vhost: memory sync fixes

This fixes two bugs related to memory sync during
migration:
    - ram address calculation was missing the chunk
      address, so the wrong page was dirtied
    - one after last was used instead of the
      end address of a region, which might overflow to 0
      and cause us to skip the region when the region ends at
      ~0x0ull.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
