Monitor: Convert do_migrate() to cmd_new_ret()

While there I'm also dropping a unneeded else clause (the last
one in the function).

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/migration.h b/migration.h
index 9345d97..385423f1 100644
--- a/migration.h
+++ b/migration.h
@@ -52,7 +52,7 @@
 
 void qemu_start_incoming_migration(const char *uri);
 
-void do_migrate(Monitor *mon, const QDict *qdict, QObject **ret_data);
+int do_migrate(Monitor *mon, const QDict *qdict, QObject **ret_data);
 
 int do_migrate_cancel(Monitor *mon, const QDict *qdict, QObject **ret_data);