tests/qtest/usb-hcd-xhci-test: Check availability of devices before using them

The "usb-uas" and "usb-ccid" might not be compiled into the QEMU binary,
so let's better check first whether they are available.

Message-Id: <20230822163024.61529-1-thuth@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Thomas Huth <thuth@redhat.com>
1 file changed