sgi/boot: use -v parameter to pass the location of virtio image

Instead of specifying busybox as the file system type, use the -v
parameter to pass the path to the virtio image.

Change-Id: Ia674929b8ed03695ec15d6e72e254090287cef27
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>

sgi/boot.sh

@@ -98,8 +98,10 @@ done
 __parse_params_validate
 
 platform_dir="platforms/$platform"
+disk_image="$platform/grub-busybox.img"
+
 pushd $platform_dir
-set -- "$@" "-f" "busybox"
+set -- "$@" "-v" "../../../../output/$disk_image"
 source ./run_model.sh
 
 # if not model failed to start, return