slirp: fix CharDriver breakage

SLIRP expects a CharBackend as the third argument to slirp_add_exec,
but net/slirp.c was passing a CharDriverState.  Fix this to restore
guestfwd functionality.
Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

net/slirp.c

@@ -763,8 +763,7 @@ static int slirp_guestfwd(SlirpState *s, const char *config_str,
             return -1;
         }
 
-        if (slirp_add_exec(s->slirp, 3, qemu_chr_fe_get_driver(&fwd->hd),
-                           &server, port) < 0) {
+        if (slirp_add_exec(s->slirp, 3, &fwd->hd, &server, port) < 0) {
             error_report("conflicting/invalid host:port in guest forwarding "
                          "rule '%s'", config_str);
             g_free(fwd);