User Networking: Enable removal of redirections

Using the new host_net_redir command you can easily create redirections
on the fly while your VM is running.

While that's great, it's missing the removal of redirections, in case you
want to have a port closed again at a later point in time.

This patch adds support for removal of redirections.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>