virtio-pci: Send qapi events when the virtio-mem size changes

Let's register the notifier and trigger the qapi event with the right
device id.

MEMORY_DEVICE_SIZE_CHANGE is similar to BALLOON_CHANGE, however on a
memory device level.

Don't unregister the notifier (we neither have finalize() nor unrealize()
for VirtIOPCIProxy, so it's not that simple to do it) - both devices are
expected to vanish at the same time.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200626072248.78761-18-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>