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: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>