storage/lvm: update volume.size attribute on resize

When volume is resized, underlying LVM is adjusted if exists. For
persistent volumes it is enough, as generally the driver prefers
underlying LVM size than internal attribute. But volatile volume is
removed at vm shutdown, so that value is lost.

Fix it by updating Volume.size attribute too, to be updated in qubes.xml
later too.

Fixes QubesOS/qubes-issues#5645

qubes/storage/lvm.py

@@ -665,6 +665,8 @@ class ThinVolume(qubes.storage.Volume):
         elif self.save_on_stop or not self.snap_on_start:
             cmd = ['extend', self._vid_current, str(size)]
             yield from qubes_lvm_coro(cmd, self.log)
+
+        self._size = size
         yield from reset_cache_coro()
 
     @asyncio.coroutine