slirp: Honour vlan/stack in hostfwd_remove commands

slirp: Honour vlan/stack in hostfwd_remove commands

The hostfwd_add and hostfwd_remove monitor commands allow the user
to optionally specify a vlan/stack tuple. hostfwd_add honours this,
but hostfwd_remove does not (it looks up the tuple but then ignores
the SlirpState it has looked up and always uses the first stack
in the list anyway). Correct this to honour what the user requested.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>