core: Fix NPE when committing a snapshot.

A regression caused by commit 268152abf19fbd64fd867bc5e5e40a57996aedc7
made the commit snapshot to throw NPE when commiting a snapshot, since
the new snapshot id has been created twice, once for the snapshots table
and once for the vm_snapshot_id of the image.

The fix set the parameter only once.

Change-Id: If423dd90739cd14a66a7a62399f56465a2e30021
Bug-Url: https://bugzilla.redhat.com/981367
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>