Code review comment for ~pieq/plainbox-provider-checkbox:1660386-mount-drive-before-storage-tests

Thanks a lot for your comments, Jeff!

I adapted a little bit the find_largest_partition (we use this test on laptop/desktops, so no need to check for LVM on unmounted drives I think).

I also got rid of the partition table check using parted, as it became useless when using find_largest_partition. If there is (largest) partition, we're all good, if not we just display an error and some help.