bugfix: UpdateVmDevice QoS

QoS was not handled in method Vm._updateInterfaceDevice.

Inbound/outbound is kept in current state if related parameter is missing in
specParams. It is deleted if it is in specParams but empty ({'inbound':{}}).

Change-Id: I43a383b2a9cf96366927beebf63f1344027169fb
Bug-Url: https://bugzilla.redhat.com/1002300
Signed-off-by: Petr Sebek <psebek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19545
Reviewed-by: Eyal Edri <eedri@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>