spapr: Split DRC release from DRC detach

spapr: Split DRC release from DRC detach

spapr_drc_detach() is called when qemu generic code requests a device be
unplugged.  It makes a number of tests, which could well delay further
action until later, before actually detach the device from the DRC.

This splits out the part which actually removes the device from the DRC
into spapr_drc_release().  This will be useful for further cleanups.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>