aemfvp-a-rme: add realm guest boot script

adding use case util script for launching a realm guest on a running FVP
instance. use case has following steps:
- launch lkvm guest with lkvm run --realm
- wait for buildroot login to appear.
- stop lkvm guest on successfull boot of realm machine.

Signed-off-by: nitin sharma <nitin.sharma3@arm.com>
Change-Id: Ib6cc7eff107a48c5cc74e2afbbb1fffc37a82de8