qobject: let object_property_get_str() use new API

We can simplify object_property_get_str() using the new
qobject_get_try_str().

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-5-peterx@redhat.com>
[eblake: rebase context of qobject_to() macro]
Signed-off-by: Eric Blake <eblake@redhat.com>