tests: validate an XML config with USB vendor/product set

tests: validate an XML config with USB vendor/product set

The USB vendor/product is usually translated into a device/bus at
startup using the hostdev logic. We don't run the latter in the
unit test suite, but we can fake it by hardcoding a translation.
This demonstrates that we format the command line with the normal
device/bus properties, even when vendor/product is set.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>