Add functionality to connect to VM microdroid shell

Added the vm-connect option to enable connecting to the microdroid running
instance/instances shell. User can connect to a running microdroid instance
by passing the CID to the vm-connect option, i.e.:

        ./run_microdroid_demo.sh vm-connect <CID number>

If no CID is specified, the script will search for the available CIDs. If only
one is found, it will automatically connect to it. If multiple instances are
found, the script will prompt the available CIDs for the user to select one of
them to connect to.

Signed-off-by: Ahmed Genidi <ahmed.genidi@arm.com>