qubesvm: improve error message about same-pool requirement

Make it clear that volume creation fails because it needs to be in the
same pool as its parent. This message is shown in context of `qvm-create
-p root=MyPool` for example and the previous message didn't make sense
at all.

Fixes QubesOS/qubes-issues#3438

qubes/vm/qubesvm.py

@@ -1990,7 +1990,9 @@ def _patch_pool_config(config, pool=None, pools=None):
         if not is_snapshot:
             config['pool'] = str(pools[name])
         else:
-            msg = "Can't clone a snapshot volume {!s} to pool {!s} " \
+            msg = "Snapshot volume {0!s} must be in the same pool as its " \
+                  "origin ({0!s} volume of template)," \
+                  "cannot move to pool {1!s} " \
                 .format(name, pools[name])
             raise qubes.exc.QubesException(msg)
     return config