Fix revisions_to_keep volume property when creating VM in a specific pool

The revisions_to_keep should be inherited from the pool by default (or
whatever else logic is in the storage pool driver). When creating VM in
a specific pool, volumes config is re-initialized to include right
defaults. But the config cleaning logic in `_clean_volume_config()`
failed to remove revisions_to_keep property initialized by the default
pool driver. This prevented new pool driver to apply its own default
logic.

An extreme result was inability to create a VM in 'file' pool at all,
because it refuses any revisions_to_keep > 1, and the default LVM
pool has revisions_to_keep = 2.

qubes/vm/qubesvm.py

@@ -2262,7 +2262,7 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
 
 def _clean_volume_config(config):
     common_attributes = ['name', 'pool', 'size',
-                         'revisions_to_keep', 'rw', 'snap_on_start',
+                         'rw', 'snap_on_start',
                          'save_on_stop', 'source']
     return {k: v for k, v in config.items() if k in common_attributes}