um: virtio: Implement VHOST_USER_PROTOCOL_F_REPLY_ACK

Implement the VHOST_USER_PROTOCOL_F_REPLY_ACK extension for both
slave requests (previous patch) where we have to reply and our
own requests where it helps understand if the slave failed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>