slirp: Do not allow to remove non-hostfwd sockets

Prevent that the users accidentally shoots down dynamic sockets. This
allows to remove looping for removals as there can now only be one
match.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
