usb-host: switch to LIBUSB_API_VERSION

usb-host: switch to LIBUSB_API_VERSION

libusbx doesn't exist any more, the fork got merged back to libusb.  So
stop using LIBUSBX_API_VERSION and use LIBUSB_API_VERSION instead.  For
backward compatibility alias LIBUSB_API_VERSION to LIBUSBX_API_VERSION
in case we figure LIBUSB_API_VERSION isn't defined.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20170403105238.23262-1-kraxel@redhat.com