tests/qtest/usb-hcd-uhci-test: Skip test if UHCI controller is not available

tests/qtest/usb-hcd-uhci-test: Skip test if UHCI controller is not available

The test is already fenced with CONFIG_USB_UHCI in meson.build, but in
case we build the ppc or mips targets in parallel, this config switch
is still set in "config_all_devices" and thus the test is still run.
Thus we need an explicit additional check here before adding the tests
to the test plan.

Message-Id: <20230512124033.502654-13-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>