Avoid crashing on multiple -incoming

Passing multiple -incoming options used to crash qemu (due to
an invalid state transition incoming->incoming).  Instead we now
take the last -incoming option, e.g.:

qemu-system-x86_64 -nographic -incoming tcp::4444 -incoming defer

ends up doing the defer.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/vl.c b/vl.c
index 75ec292..74c2681 100644
--- a/vl.c
+++ b/vl.c
@@ -3618,8 +3618,10 @@
                 }
                 break;
             case QEMU_OPTION_incoming:
+                if (!incoming) {
+                    runstate_set(RUN_STATE_INMIGRATE);
+                }
                 incoming = optarg;
-                runstate_set(RUN_STATE_INMIGRATE);
                 break;
             case QEMU_OPTION_nodefaults:
                 has_defaults = 0;