Fix for VG/LV names containing hyphens and/or dots

This patch both adds hyphens and dots to the allowed characters for a variable name in the vgcfgbackup parser, and correctly translates hyphenated VG/LV names into COW device names.

This pull request supercedes https://github.com/mpalmer/lvmsync/pull/16, and I will be closing that pull request momentarily with a link to this one.