CP-5600: Use /dev/mapper names for LVM names.

Former LVM /dev/ naming end up constructing devices in /dev/mapper while
current code put only symlinks here causing snapshot parent names to be
something useless like dm-XX which is not persistent.
The patch instead of getting the real device name try to get a compatible
name.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>