Support creating guest with USB host device in config file.
Currently libxl only supports xen PV guest, and only supports
specifying USB host device by 'bus number' and 'device number',
for example:

    <hostdev mode='subsystem' type='usb' managed='no'>
      <source>
        <address bus='1' device='3'/>
      </source>
    </hostdev>
Signed-off-by: NChunyan Liu <cyliu@suse.com>
Signed-off-by: NJim Fehlig <jfehlig@suse.com>