tests: Add unit tests for the VM Generation ID feature

The following tests are implemented:
* test that a GUID passed in by command line is propagated to the guest.
  Read the GUID from guest memory
* test that the "auto" argument to the GUID generates a valid GUID, as
  seen by the guest.
* test that a GUID passed in can be queried from the monitor

  This patch is loosely based on a previous patch from:
  Gal Hammer <ghammer@redhat.com>  and Igor Mammedov <imammedo@redhat.com>

Signed-off-by: Ben Warren <ben@skyportsystems.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
