Description of problem:
It is possible to start a delete operation for single snapshot multiple times.

Version-Release number of selected component (if applicable):


How reproducible:
always

Steps to Reproduce:
1. create/have at least one VM snapshot
2. try to delete the snapshot
3. repeat 2

Actual results:
It is possible to start multiple snapshot delete operations and eventually Virt Manager will return this traceback:

Error deleting snapshot 'pre post F21 mas rebuild update': Domain snapshot not found: no domain snapshot with matching name 'pre post F21 mas rebuild update'

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 91, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 127, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 183, in delete
    self._backend.delete()
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 4702, in delete
    if ret == -1: raise libvirtError ('virDomainSnapshotDelete() failed')
libvirtError: Domain snapshot not found: no domain snapshot with matching name 'pre post F21 mas rebuild update'

Also the snapshot appears to be gone.

Expected results:
It is not possible to start multiple delete operations for a single VM snapshot.