core: use DiskImage.copyOf in getAllDisks query

GetAllDisksByStorageDomainIdQuery:
add a copy of the active DiskImage to snapshots list
[adding it as is causes infinte recursion in DiskImage->hashCode()]

Change-Id: Ieef49f05980cdac8a5e3fbc3652a2ed261c6a8b0
Bug-Url: https://bugzilla.redhat.com/951909
Signed-off-by: Daniel Erez <derez@redhat.com>