virtio-mem: Unplug subblocks right-to-left

We unplug blocks right-to-left, let's also unplug subblocks within a block
right-to-left.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-14-david@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>