virtio_balloon: return the amount of freed memory from leak_balloon()
This value would be useful in the next patch to provide the amount of
the freed memory for OOM killer.
Signed-off-by: Raushaniya Maksudova <rmaksudova@parallels.com> Signed-off-by: Denis V. Lunev <den@openvz.org> CC: Rusty Russell <rusty@rustcorp.com.au> CC: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>