core: register RemoveSnapshotCommandCallback only for a running VM

RemoveSnapshotCommand - registering RemoveSnapshotCommandCallback
for a non running VM is redundant and lead to an exception as described
in the bug. Hence, adding the callback only for a running VM.

Change-Id: I1f2440a6860dbdd5ece5c3979c710e90ccaa3929
Bug-Url: https://bugzilla.redhat.com/1148710
Signed-off-by: Daniel Erez <derez@redhat.com>