Disallow outward migration while awaiting incoming migration

QEMU will assert if you attempt to start an outgoing migration on
a QEMU that's sitting waiting for an incoming migration (started
with -incoming), so disallow it with a proper error.

(This is a fix for https://bugzilla.redhat.com/show_bug.cgi?id=1086987 )

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration.c

@@ -419,6 +419,11 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
         return;
     }
 
+    if (runstate_check(RUN_STATE_INMIGRATE)) {
+        error_setg(errp, "Guest is waiting for an incoming migration");
+        return;
+    }
+
     if (qemu_savevm_state_blocked(errp)) {
         return;
     }