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