virtio-mem: Update state to match bitmap as soon as it's been migrated

virtio-mem: Update state to match bitmap as soon as it's been migrated

It's cleaner and future-proof to just have other state that depends on the
bitmap state to be updated as soon as possible when restoring the bitmap.

So factor out informing RamDiscardListener into a functon and call it in
case of early migration right after we restored the bitmap.

Message-ID: <20230926185738.277351-16-david@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>