hw/ppc/spapr.c: recover pending LMB unplug info in spapr_lmb_release

hw/ppc/spapr.c: recover pending LMB unplug info in spapr_lmb_release

When a LMB hot unplug starts, the current DRC LMB status is stored at
spapr->pending_dimm_unplugs QTAILQ. This queue isn't migrated, thus
if a migration occurs in the middle of a LMB unplug the
spapr_lmb_release callback will lost track of the LMB unplug progress.

This patch implements a new recover function spapr_recover_pending_dimm_state
that is used inside spapr_lmb_release to recover this DRC LMB release
status that is lost during the migration.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
[dwg: Minor stylistic changes, simplify error handling]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>