bus: do not unref hotplug handler

bus: do not unref hotplug handler

Apparently, none of the bus owner give a reference to the hotplug
handler property, do not unref it on bus release.

Furthermore, a bus is allowed to be its own hotplug handler, which can
be seen in qbus_set_bus_hotplug_handler() function. However, in this
case, the reference can't be given to the property, or this will create
a cyclic dependency and the bus will never be free.

Each bus owner should manage the lifecycle of the hotplug handler.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>