core: Improve performance of updateAllDiskImageDynamicWithDiskIdByVmId

updateAllDiskImageDynamicWithDiskIdByVmId is running an update on the
disk_image_dynamic table, using a subquery on the images table.
Since images table does not have an index on image_group_id, and images
can be a rather big table, this patch adds such an index.

Change-Id: I1610d633dde009f55485e3081f4094d60cd25e09
Bug-Url: https://bugzilla.redhat.com/1155153
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>