core: DiskImageDAO - introduce getAllSnapshotsForLeaf

Introducing a recursive stored procedure for retrieving
a snapshot chain by a leaf image. It is required for
ensuring data integrity when querying GetAllDisksByVmId
(i.e. avoid race while removing a disk/etc).

Change-Id: Ib6068be44fd5e5c47eed2064d87f506d3e3be444
Bug-Url: https://bugzilla.redhat.com/1113087
Signed-off-by: Daniel Erez <derez@redhat.com>