CP-5600: Do not use full paths for lvm executable.

Moved to /sbin in CentOS 6.4 so avoid to use full paths that does not work.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
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>
Merge pull request #44 from freddy77/lvm_fixes

Lvm fixes

Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>