Support hot attach/detach a USB host device to guest.
Currently libxl only supports xen PV guest, and only
supports specifying USB host device by 'bus number'
and 'device number', for example:
usb.xml:
<hostdev mode='subsystem' type='usb' managed='no'>
<source>
<address bus='1' device='3'/>
</source>
</hostdev>
#xl attach-device dom usb.xml
#xl detach-device dom usb.xml
Signed-off-by: Chunyan Liu <cyliu@suse.com> Signed-off-by: Jim Fehlig <jfehlig@suse.com>