At least Xen supports backend drivers in another domain (aka "driver
domain"). This patch introduces an XML config option for specifying the
backend domain name for <disk> and <interface> devices.  E.g.

  <disk>
    <backenddomain name='diskvm'/>
    ...
  </disk>
  <interface type='bridge'>
    <backenddomain name='netvm'/>
    ...
  </interface>

In the future, same option will be needed for USB devices (hostdev
objects), but for now libxl doesn't have support for PVUSB.
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>