tests/util/grub-shell-luks-tester: Find cryptodisk by UUID

tests/util/grub-shell-luks-tester: Find cryptodisk by UUID

GRUB has the capability to search all the disks for a cryptodisk of a
given UUID. Use this instead of hardcoding which disk is the cryptodisk,
which can change when devices are added or removed, or potentially when
QEMU is upgraded. This can not be done for the detached header tests
because the header contains the UUID.

Also, capitalize comment lines for consistency.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>