tests/acceptance: Specify arch for QueryCPUModelExpansion

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 <[email protected]>
Reviewed-by: Wainer dos Santos Moschetta <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Cleber Rosa <[email protected]>
Message-Id: <20190918070654[email protected]>
Signed-off-by: Cleber Rosa <[email protected]>