tools/qvm-start: fix starting a VM with cdrom image

1. Output of `losetup` command contains `\n` - strip it.
2. Provide read-only option - if device info hasn't propagated to qubesd
yet, it will not be set automatically.

Fixes QubesOS/qubes-issues#3146

qubesadmin/tests/tools/qvm_start.py

@@ -81,7 +81,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             ('some-vm', 'admin.vm.Start', None, None)] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'dom0+sr0',
-                b'devtype=cdrom persistent=True')] = b'0\x00'
+                b'devtype=cdrom persistent=True read-only=True')] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Set.persistent', 'dom0+sr0',
             b'False')] = b'0\x00'
@@ -101,7 +101,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             ('some-vm', 'admin.vm.Start', None, None)] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'dom0+sdb1',
-                b'devtype=disk persistent=True')] = b'0\x00'
+                b'devtype=disk persistent=True read-only=False')] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Set.persistent', 'dom0+sdb1',
             b'False')] = b'0\x00'
@@ -121,7 +121,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             ('some-vm', 'admin.vm.Start', None, None)] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'dom0+sdb1',
-                b'devtype=disk persistent=True')] = b'0\x00'
+                b'devtype=disk persistent=True read-only=False')] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Set.persistent', 'dom0+sdb1',
             b'False')] = b'0\x00'
@@ -142,7 +142,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             ('some-vm', 'admin.vm.Start', None, None)] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'dom0+loop12',
-                b'devtype=cdrom persistent=True')] = b'0\x00'
+                b'devtype=cdrom persistent=True read-only=True')] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Set.persistent', 'dom0+loop12',
             b'False')] = b'0\x00'
@@ -168,7 +168,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             ('some-vm', 'admin.vm.Start', None, None)] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'other-vm+loop7',
-                b'devtype=cdrom persistent=True')] = b'0\x00'
+                b'devtype=cdrom persistent=True read-only=True')] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Set.persistent',
             'other-vm+loop7',
@@ -193,7 +193,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             b'some-vm class=AppVM state=Running\n'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'other-vm+loop7',
-                b'devtype=cdrom persistent=True')] = b'0\x00'
+                b'devtype=cdrom persistent=True read-only=True')] = b'0\x00'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.Start', None, None)] = \
             b'2\x00QubesException\x00\x00An error occurred\x00'
@@ -214,7 +214,7 @@ class TC_00_qvm_start(qubesadmin.tests.QubesTestCase):
             b'some-vm class=AppVM state=Running\n'
         self.app.expected_calls[
             ('some-vm', 'admin.vm.device.block.Attach', 'other-vm+loop7',
-                b'devtype=cdrom persistent=True')] = \
+                b'devtype=cdrom persistent=True read-only=True')] = \
             b'2\x00QubesException\x00\x00An error occurred\x00'
         retcode = qubesadmin.tools.qvm_start.main([
             '--cdrom=other-vm:loop7',

qubesadmin/tools/qvm_start.py

@@ -118,13 +118,15 @@ def get_drive_assignment(app, drive_str):
         except subprocess.CalledProcessError:
             raise qubesadmin.exc.QubesException(
                 'Failed to setup loop device for %s', ident)
+        loop_name = loop_name.strip()
         assert loop_name.startswith(b'/dev/loop')
         ident = loop_name.decode().split('/')[2]
         # FIXME: synchronize with udev + exposing device in qubesdb
 
-    options = {}
-    if devtype:
-        options['devtype'] = devtype
+    options = {
+        'devtype': devtype,
+        'read-only': devtype == 'cdrom'
+    }
     assignment = qubesadmin.devices.DeviceAssignment(
         backend_domain,
         ident,