extnet vdsm hook
=================================
This hook allows to override Engine's choice of to which bridge device should a
vNIC be connected.

Installation:
* Use the engine-config to append the appropriate custom property:

  sudo engine-config -s CustomDeviceProperties=
  '{type=interface;prop={extnet=^[a-zA-Z0-9_ ---]+$}}'

* Verify that the extnet custom propertes were properly added:
	sudo engine-config -g CustomDeviceProperties

Usage:
Define a vNIC profile and set one of its custom properites to "extnet" with
the value of the name of a libvirt network that is managed outside of oVirt.
Then, attach the defined profile to a vNIC. When the VM is run, the relevant
vNIC would be attached to the forced external network network instead of the
oVirt network specified by Engine.

For example, to use an openvswitch bridge, first virsh net-define
    <network>
      <name>ovs-net</name>
      <forward mode='bridge'/>
      <bridge name='ovsbr0'/>
      <virtualport type='openvswitch'/>
    </network>
and make sure the network and its bridge are up and running. Then, vNICs with
extnet=ovs-net would be connected to this network by the hook.
