com.docker.slirp: use the Active_list.Instance signature
com.docker.slirp: use the Active_list.Instance signature

Rather than repeating it inline.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,samoht/vpnkit,djs55/vpnkit