rdn1edge/run_model: remove default virtio image selection for busybox

Do not default to a virtio image for busybox filesystem. The -v
parameter is expected to set the virtio disk image to be used.

Change-Id: I994b2fa9ec283053af206d0a56931a0783f26d87
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>