spapr: Uniform DRC reset paths

spapr: Uniform DRC reset paths

DRC objects have a regular device reset method.  However, it only gets
called in the usual way for PCI DRCs.  Because of where CPU and LMB DRCs
are in the QOM tree, their device reset method isn't automatically called.
So, the machine manually registers reset handlers to call device_reset().

This patch removes the device reset method, and instead always explicitly
registers the reset handler from realize().  This means the callers don't
have to worry about the two cases, and we always get proper resets.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>