libxl: fix regression introduced by commit 4ab8cd77

libxl: fix regression introduced by commit 4ab8cd77

Commit 4ab8cd77 added a check requiring input devices to have
a bus type of VIR_DOMAIN_INPUT_BUS_USB, failing to start the
domain otherwise.  But virDomainDefParseXML adds implicit mouse
and keyboard if a graphics device is configured.  See calls to
virDomainDefMaybeAddInput.

The regression is fixed by removing the check requiring USB input
devices, and skipping non-USB input devices when populating USB
'usbdevice' in libxl_domain_build_info struct.