Instantiate a QObject* from a literal QLitObject.

LitObject only supports int64_t for now.  uint64_t and double aren't
implemented.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180305172951.2150-4-marcandre.lureau@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>