virito-mem: existing (un)plug functions are specific to Sub Block Mode (SBM)

Let's rename them accordingly. virtio_mem_plug_request() and
virtio_mem_unplug_request() will be handled separately.
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/20201112133815.13332-23-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>