webadmin: Add QoS widget to host network dialog

Added a QoS widget to the edit host network dialog, which allows users
to override the QoS configuration on a specific host interface.

Also refactored some more NetworkQoSModel-related code, some for reuse
purposes and some to encapsulate changeability/availability state
management in BaseNetworkQosModel.

Change-Id: I0f622b23f86a8e19d373f2210e71804006e70c5f
Signed-off-by: Lior Vernia <lvernia@redhat.com>