virtio-mem: simplify high-level plug handling in Sub Block Mode

Let's simplify high-level memory block selection when plugging in Sub
Block Mode.

No need for two separate loops when selecting memory blocks for plugging
memory. Avoid passing the "online" state by simply obtaining the state
in virtio_mem_sbm_plug_any_sb().

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20210602185720.31821-4-david@redhat.com


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