qemuxmlconftest: add tests for new hardware UUID (hwuuid) element

qemuxmlconftest: add tests for new hardware UUID (hwuuid) element

Add new 'hwuuid' and 'hwuuid-smbios-uuid-match' tests to verify that the new
hardware UUID (hwuuid) element is working as intended.

In particular the 'hwuuid-smbios-uuid-match' test is designed to ensure that
the hwuuid and smbios UUIDs are always identical, similar to the existing logic
works for the domain UUID and the smbios UUID.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>