Add strict name validation for the LVM backend

Currently we don't enforce name validation for the LVM backend, on the
idea that LVM itself will reject invalid names and we catch those
errors.

However, recent LVM documents the accepted VG/LV name space, so it's
easy to add this in the LVM backend code.

In addition, the patch changes some hardcoded /dev/ constructions with
utils.PathJoin().

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>