slirp: Rework monitor commands for host forwarding

Improve the monitor interface for adding and removing host forwarding
rules by splitting it up in two commands and rename them to hostfwd_add
and hostfwd_remove. Also split up the paths taken for legacy -redir
support and the monitor add command as the latter will be extended later
on.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/vl.c b/vl.c
index 4f9c521..23add31 100644
--- a/vl.c
+++ b/vl.c
@@ -5319,7 +5319,7 @@
                 break;
 #endif
             case QEMU_OPTION_redir:
-                net_slirp_redir(NULL, optarg, NULL);
+                net_slirp_redir(optarg);
                 break;
 #endif
             case QEMU_OPTION_bt: