backend: prevent live storage migration on live snapshot failure

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1018876
Change-Id: I7c69f663836e74691c968f4a15c5f3012479a8b0
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
backend: unlock images when GetImageInfo fails

In BaseImagesCommand.endSuccessfully() we try to update the image
information contacting VDSM. If VDSM is down or for any other error
we should fail gracefully (logging) and release the locks on the
image.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1018876
Change-Id: I980e60ba5ad4fcc07752d065c3eaca03af025896
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>