storage: make Volumes sortable

But do not fetch any additional info just for this purpose.

qubesadmin/storage.py

@@ -93,6 +93,16 @@ class Volume(object):
             return self.pool == other.pool and self.vid == other.vid
         return NotImplemented
 
+    def __lt__(self, other):
+        # pylint: disable=protected-access
+        if isinstance(other, Volume):
+            if self._vm and other._vm:
+                return (self._vm, self._vm_name) < (other._vm, other._vm_name)
+            elif self._vid and other._vid:
+                return (self._pool, self._vid) < (other._pool, other._vid)
+        return NotImplemented
+
+
     @property
     def name(self):
         '''per-VM volume name, if available'''