fix(dmsquash-live): run checkisomd5 on correct device

fix(dmsquash-live): run checkisomd5 on correct device

When the new grub2 iso is written to a usb drive the disk label points
to a partition that does not include the full iso image. This causes
checkisomd5 to run with the wrong data and it fails.

This patch adds a check that will test to see if there is a parent
device that is a disk, and to run checkisomd5 on it instead of on the
partition pointed to by the label.

When running from an iso this will return the original
/dev/disk/by-label/ path, and when running from a usb drive it will
return the parent device (eg. /dev/sda).

Resolves: rhbz#2107858