tools/qvm-start: add naive waiting for cdrom device

When starting a VM with --cdrom=some-vm:/some/path/to.iso, it can be
started only when loop device matching the path is available. For now,
add naive waiting (while ... sleep(1)) for it. Later it might worth
converting it to events handling.

qubesadmin/tools/qvm_start.py

@@ -24,6 +24,8 @@ import sys
 
 import subprocess
 
+import time
+
 import qubesadmin.devices
 import qubesadmin.exc
 import qubesadmin.tools
@@ -121,7 +123,17 @@ def get_drive_assignment(app, drive_str):
         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
+        # wait for device to appear
+        # FIXME: convert this to waiting for event
+        timeout = 10
+        while isinstance(backend_domain.devices['block'][ident],
+                qubesadmin.devices.UnknownDevice):
+            if timeout == 0:
+                raise qubesadmin.exc.QubesException(
+                    'Timeout waiting for {}:{} device to appear'.format(
+                        backend_domain.name, ident))
+            timeout -= 1
+            time.sleep(1)
 
     options = {
         'devtype': devtype,