doc: Add virtualization section for the actuation use-case

The Use-Cases section for the Actuation Demo mentions the Virtualization
Architecture but omits some details about how to access the DomU
console.

Label the existing instructions "Baremetal Architecture" and add a short
"Virtualization Architecture" section below, referencing the above.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>