conf: Introduce new hostdev device type mdev

conf: Introduce new hostdev device type mdev

A mediated device will be identified by a UUID (with 'model' now being
a mandatory <hostdev> attribute to represent the mediated device API) of
the user pre-created mediated device. We also need to make sure that if
user explicitly provides a guest address for a mdev device, the address
type will be matching the device API supported on that specific mediated
device and error out with an incorrect XML message.

The resulting device XML:
<devices>
  <hostdev mode='subsystem' type='mdev' model='vfio-pci'>
    <source>
      <address uuid='c2177883-f1bb-47f0-914d-32a22e3a8804'>
    </source>
  </hostdev>
</devices>

Signed-off-by: Erik Skultety <eskultet@redhat.com>