Double comma + losetup to access the disk

Adding 'disk = [ 'file:/home/avsm/src/git/avsm/mirage-skeleton/block/disk.img,,xvda1,w']' to the file block_test.xl doesnot do the job. it can be done with losetup to point to a directory: 'disk = [ '/dev/loop0,,xvda1,w']'

(and it should be ',,' between the directory and 'xvda1')