scripts/run.py: Add wait feature

It simply tells QEMU not to start OSv at startup.

To continue the execution, it's possible to use either the QEMU monitor
or a GDB remote connection.

Signed-off-by: Raphael S. Carvalho <raphael.scarv@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>