core: ImagesHandler - lock multiple disks snapshots in one transaction

this patch adds support and usage of locking the snapshots of multiple
disks in one transaction (including support for using compensation)

Change-Id: If873e23983757e9e9bb41a760d07779fc1512cd4
Related To: https://bugzilla.redhat.com/show_bug.cgi?id=1028768
Signed-off-by: Liron Aravot <laravot@redhat.com>