Enhancing devices search ordering.

Enhancing devices search ordering.

E.g. prefer mounting /dev/sdb1 rather than /dev/sdb.

The isohybrid feature makes both valid devices for being searched for a live
filesystem image. Previous code mounted /dev/sdb on /live/image which makes it
hard to create a second partition on the USB stick.

This brings the need to explicitly check whether the full device is in the
blacklist before even considering its subdevices: the previous blacklist
checking code worked because the full device was always considered before its
subdevices, which is not true anymore.