virtio-mem: Exclude unplugged memory during migration

The content of unplugged memory is undefined and should not be migrated,
ever. Exclude all unplugged memory during precopy using the precopy notifier
infrastructure introduced for free page hinting in virtio-balloon.

Unplugged memory is marked as "not dirty", meaning it won't be
considered for migration.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200626072248.78761-21-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>