virtio-mem: Offline and remove completely unplugged memory blocks

virtio-mem: Offline and remove completely unplugged memory blocks

Let's offline+remove memory blocks once all subblocks are unplugged. We
can use the new Linux MM interface for that. As no memory is in use
anymore, this shouldn't take a long time and shouldn't fail. There might
be corner cases where the offlining could still fail (especially, if
another notifier NACKs the offlining request).

Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Tested-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-10-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>