tc: Using RoS path for virtio

Adding a new parameter for the virtio image path for the fvp.
This will unify the scripts for tc4 and other tc variants.
tc4 will use the new one, others use the old one:
	-TC4: cpnss.ros.virtio_blockdevice.image_path
	-TCx: board.virtioblockdevice.image_path

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