core: GetAllVmSnapshotsFromConfigurationByVmIdQuery

Adding a new query for fetching snapshots of a VM from
configuration (including disks list).
Needed for display relevant disks data as part of
single disk snapshot feature [1].

[1] http://www.ovirt.org/Features/Single_Disk_Snapshot

Change-Id: I23aa4de4d233fade33d2a5ea174a9a1802b49370
Signed-off-by: Daniel Erez <derez@redhat.com>