engine: Adding GetInterfacesAttachedToNetworkViaLabelQuery

A query that returns all the VdsNetworkInterface entities the network in
the parameter is attached to them via label.

Change-Id: Ib5701455e2b372e4b656c87e05781fe05a95f9f0
Bug-Url: https://bugzilla.redhat.com/1063797
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>