Use QDBusReply instead of manipulating the QDBusMessage by hand.

Not only is this neater, it fixes a screwup in
1be686159ce23a4fc683b1c17926f923ff957513. QVariant apparently doesn't want to
implicitly convert the QVariantMap via toMap, but instead silently returns an
empty map.

Change-Id: I0a71bf54a0d69a7f2da6a26fa77c53f7d3d9a588