restapi: Support ;current on cdroms collection

Add support for displaying current CDs (as opposed the configured)
using the ;current matrix parameter on the BackendCdRomsResource.

So far it was possible to get current CD only per individual VM (using
BackendCdRomResource)

Change-Id: Iaa14b3ceb09ab8d824fa9fae0ef3eb7d4df7f855
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1092070
Signed-off-by: Martin Betak <mbetak@redhat.com>