core: add disk list member to Snapshot entity

Adding diskImages list member to snapshot business entity.
This member will be used for retrieving disks of a specific
snapshot (needed for single disk snapshot feature [1])

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

Change-Id: I498f2dc8a5e6d5311b9b4b7fa17c585c293d734e
Signed-off-by: Daniel Erez <derez@redhat.com>