nodedev: introduce concept of 'active' node devices

nodedev: introduce concept of 'active' node devices

we will be able to define mediated devices that can be started or
stopped, so we need to be able to indicate whether the device is active
or not, similar to other resources (storage pools, domains, etc.)

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>