virtio-mem-pci: Device unplug support

virtio-mem-pci: Device unplug support

Let's support device unplug by forwarding the unplug_request_check()
callback to the virtio-mem device.

Further, disallow changing the requested-size once an unplug request is
pending.

Disallowing requested-size changes handles corner cases such as
(1) pausing the VM (2) requesting device unplug and (3) adjusting the
requested size. If the VM would plug memory (due to the requested size
change) before processing the unplug request, we would be in trouble.

Message-ID: <20230711153445.514112-8-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>