core: Add Vnic Profile support to RemoveNetworkQoS

When removing a NetworkQoS we must set all the Vnic Profiles which used
that NetworkQoS to use null NetworkQoS

Added support for getting a list of all Vnic Profiles using a specific
NetworkQoS from UI (in order to alert the user and show him a list)

Change-Id: I401cff6b67b5fe62303323849f11bd9d03892201
Signed-off-by: Ofri Masad <omasad@redhat.com>
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>
Signed-off-by: Ofri Masad <omasad@redhat.com>