virtio-mem: nb_sb_per_mb and subblock_size are specific to Sub Block Mode (SBM)

Let's rename to "sbs_per_mb" and "sb_size" and move accordingly.
Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-21-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>