slirp: fix guestfwd id

When using -net user,guestfwd=... Qemu immediately complains about the id
being in invalid format. This is because we pass in an id that contains a
colon, while the id restrictions don't allow colons.

This patch changes the colon into a dot, making guestfwd work again.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
