Instantiate a QObject* from a literal QLitObject.

LitObject only supports int64_t for now.  uint64_t and double aren't
implemented.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20180305172951.2150-4-marcandre.lureau@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>