Move container-specific uevents from the core hotplug code to the
container scan handler's .attach() and .detach() callbacks.

This way the core will not have to special-case containers and
the uevents will be guaranteed to happen every time a container
is either scanned or trimmed as appropriate.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>