tests/acceptance: Specify arch for QueryCPUModelExpansion

At the moment this test runs on whatever the host arch is.  But it looks
for 'unavailable-features' which is an x86 specific cpu property.  Tag it
to always use qemu-system-x86_64.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20190918070654.19356-1-david@gibson.dropbear.id.au>
Signed-off-by: Cleber Rosa <crosa@redhat.com>