slirp: Clean up net_slirp_hostfwd_remove()'s use of get_str_sep()

get_str_sep() can fail, but net_slirp_hostfwd_remove() doesn't check.
Works, because it initializes buf[] to "", which get_str_sep() doesn't
touch when it fails.  Coverity doesn't like it, and neither do I.

Change it to work exactly like slirp_hostfwd().

Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
