spapr.c: handle dev->id in spapr_memory_unplug_rollback()

spapr.c: handle dev->id in spapr_memory_unplug_rollback()

As done in hw/acpi/memory_hotplug.c, pass an empty string if dev->id
is NULL to qapi_event_send_mem_unplug_error() to avoid relying on
a behavior that can be changed in the future.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210907004755.424931-3-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>