slirp: Give error message if hostfwd_add/remove for unrecognized vlan/stack

slirp: Give error message if hostfwd_add/remove for unrecognized vlan/stack

If the user specified a (vlan ID, slirp stack name) tuple in a monitor
hostfwd_add/remove command and we can't find it, give the user an
error message rather than silently doing nothing.

This brings this error case in slirp_lookup() into line with the
other two.

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