virtio-mem: generalize check for added memory

Let's check by traversing busy system RAM resources instead, to avoid
relying on memory block states.

Don't use walk_system_ram_range(), as that works on pages and we want to
use the bare addresses we have easily at hand.

This is a preparation for Big Block Mode (BBM), which won't have memory
block states.

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/20201112133815.13332-12-david@redhat.com


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