storage/file: get volume size from the actual image file size

Don't realy on a volume configuration only, it's easy to miss updating
it. Specifically, import_volume() function didn't updated the size based
on the source volume.
The size that the actual VM sees is based on the
file size, and so is the filesystem inside. Outdated size property can
lead to a data loss if the user perform an action based on a incorrect
assumption - like extending size, which actually will shrink the volume.

Fixes QubesOS/qubes-issues#4821

qubes/storage/file.py

@@ -28,6 +28,7 @@ import os
 import os.path
 import re
 import subprocess
+from contextlib import suppress
 
 import qubes.storage
 
@@ -394,6 +395,17 @@ class FileVolume(qubes.storage.Volume):
         iso_date = qubes.storage.isodate(seconds).split('.', 1)[0]
         return {'old': iso_date}
 
+    @property
+    def size(self):
+        with suppress(FileNotFoundError):
+            self._size = os.path.getsize(self.path)
+        return self._size
+
+    @size.setter
+    def size(self, _):
+        raise qubes.storage.StoragePoolException(
+            "You shouldn't use volume size setter, use resize method instead")
+
     @property
     def usage(self):
         ''' Returns the actualy used space '''