core: avoid db deadlock and unneeded db updates for images

When adding a VM from template (as part of pool or not),
CreateSnapshotFromTemplateCommand is being executed and when the
operation ends its end method is being called for each of the disks.

In the end method there's an attempt to unlock the template image which
causes to numerous issues:
1. There are attempts to update the template disks statuses to ok,
as the updates aren't ordered and are executed for each added vm based
on that template - db deadlocks occur.
2.Currently the template disks aren't being locked at all (which 
should be inspected regardless), so there's currently no point in unlocking them.

The fix which is introduced is to not update the image status if it's
already in the desired status, which will reduce the updates count in
general and will prevent possible db deadlocks in the provided
scenarios.

Change-Id: I26a09ce674cfefd59819f3edf95accab31f0e80e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=915753
Signed-off-by: Liron Aravot <laravot@redhat.com>