storage: pass a copy of volume_config to pool.init_volume

Avoid local modification in a pool's init_volume influence
vm.volume_config. Currently every pool driver replaces
volume_config['pool'] with a pool object (instead of name) and it leads
to confusing cases where depending on start stage, it is sometimes an
object and sometimes a string.
Additionally, some pool drivers may modify volume_config in unexpected
way - for example test pool driver removes 'pool' entry entirely. Avoid
this fragile interface by giving pool driver a copy of volume_config,
instead of vm.volume_config directly.

Note one side effect is that 'vid' (and other pool-specific parameters)
is not set into vm.volume_config directly after creating a VM, but
possibly only after loading from XML. This should not be an issue in
theory (no core code should expect it), but if some place use
volume_config instead of Volume instance for getting pool-specific
options, it should be fixed.

qubes/storage/__init__.py

@@ -423,7 +423,7 @@ class Storage:
         if 'internal' in volume_config:
             # migrate old config
             del volume_config['internal']
-        volume = pool.init_volume(self.vm, volume_config)
+        volume = pool.init_volume(self.vm, volume_config.copy())
         self.vm.volumes[name] = volume
         return volume