tests/qtest/migration: Add a check for the availability of the "pc" machine

tests/qtest/migration: Add a check for the availability of the "pc" machine

The test_vcpu_dirty_limit is the only test that does not check for the
availability of the machine before starting the test, so it fails when
QEMU has been configured with --without-default-devices. Add a check for
the "pc" machine type to fix it.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>