virtio-mem: use "unsigned long" for nr_pages when fake onlining/offlining

virtio-mem: use "unsigned long" for nr_pages when fake onlining/offlining

No harm done, but let's be consistent.

Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
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/20201112133815.13332-6-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>