qapi/qdev.json: add DEVICE_UNPLUG_GUEST_ERROR QAPI event

At this moment we only provide one event to report a hotunplug error,
MEM_UNPLUG_ERROR. As of Linux kernel 5.12 and QEMU 6.0.0, the pseries
machine is now able to report unplug errors for other device types, such
as CPUs.

Instead of creating a (device_type)_UNPLUG_ERROR for each new device,
create a generic DEVICE_UNPLUG_GUEST_ERROR event that can be used by all
guest side unplug errors in the future. This event has a similar API as
the existing DEVICE_DELETED event, always providing the QOM path of the
device and dev->id if there's any.

With this new generic event, MEM_UNPLUG_ERROR is now marked as deprecated.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210907004755.424931-6-danielhb413@gmail.com>
[dwg: Correct missing ')' in stubs/qdev.c]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>