storage: fallback kernels_dir path if there is no 'kernel' volume

Return meaningful value for kernels_dir if VM has no 'kernel' volume.
Right now it's mostly useful for tests, but could be also used for new
VM classes which doesn't have modules.img, but still use dom0-provided
kernel.

qubes/storage/__init__.py

@@ -468,8 +468,14 @@ def kernels_dir(self):
         If :py:attr:`self.vm.kernel` is :py:obj:`None`, the this points inside
         :py:attr:`self.vm.dir_path`
         '''
-        assert 'kernel' in self.vm.volumes, "VM has no kernel volume"
-        return self.vm.volumes['kernel'].kernels_dir
+        if not self.vm.kernel:
+            return None
+        if 'kernel' in self.vm.volumes:
+            return self.vm.volumes['kernel'].kernels_dir
+        return os.path.join(
+            qubes.config.qubes_base_dir,
+            qubes.config.system_path['qubes_kernels_base_dir'],
+            self.vm.kernel)
 
     def get_disk_utilization(self):
         ''' Returns summed up disk utilization for all domain volumes '''