xl: Accept a list for usbdevice in config file

xl: Accept a list for usbdevice in config file

Allow the "usbdevice" key to accept a list of USB devices, and pass
them in using the new usbdevice_list domain build element.

For backwards compatibility, still accept singleton values.

Also update the xl.cfg manpage, adding information about how to pass
through host devices.

as applied:
 - Fix trailing whitespace and wrap some lines in xl_cmdimpl.c -iwj
v2:
 - Add some verbiage to make it clear that "usb" is for emulated devices
 - Reference qemu manual for more usbdevice options

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>