tests/check-qom-proplist: Improve iterator coverage

tests/check-qom-proplist: Improve iterator coverage

The tests' "qemu-dummy" device has only class properties.  Turn one of
them into an instance property.  test_dummy_class_iterator() expects
one fewer property than test_dummy_iterator().  Rewrite
test_dummy_prop_iterator() to take expected properties as argument.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Message-Id: <20200505152926[email protected]>