storage/kernel: skip modules.img block device if kernel dir doesn't have it

Some kernels (like pvgrub2) may not provide modules.img and it isn't an
error. Don't break VM startup in that case, skip that device instead.

Fixes QubesOS/qubes-issues#3563

qubes/storage/kernels.py

@@ -105,10 +105,6 @@ class LinuxModules(Volume):
         return False
 
     def start(self):
-        path = self.path
-        if path and not os.path.exists(path):
-            raise StoragePoolException('Missing kernel modules: %s' % path)
-
         return self
 
     def stop(self):
@@ -116,12 +112,15 @@ class LinuxModules(Volume):
 
     def verify(self):
         if self.vid:
-            _check_path(self.path)
             _check_path(self.vmlinuz)
             _check_path(self.initramfs)
 
     def block_device(self):
-        if self.vid:
+        path = self.path
+        # create block device for modules.img only if:
+        # - there is kernel set for the VM
+        # - that kernel directory contains modules.img file
+        if path and os.path.exists(path):
             return super().block_device()
         return None
 

qubes/tests/storage_kernels.py

@@ -75,6 +75,8 @@ class TC_01_KernelVolumes(qubes.tests.QubesTestCase):
         self.app = TestApp()
         self.app.create_dummy_template()
         self.app.add_pool(**self.POOL_CONF)
+        os.makedirs(self.POOL_DIR + '/dummy', exist_ok=True)
+        open('/tmp/test-pool/dummy/modules.img', 'w').close()
 
     def tearDown(self):
         """ Remove the file based storage pool after testing """