test-qdev-global-props: Run tests on subprocess

test-qdev-global-props: Run tests on subprocess

There are multiple reasons for running the global property tests on a
subprocess:

* We need the global_props lists to be empty for each test case, so
  global properties from the previous test won't affect the next one;
* We don't want the qdev_prop_check_global() warnings to pollute test
  output;
* With a subprocess, we can ensure qdev_prop_check_global() is printing
  the warning messages it should.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>