CA-113322: avoid using realpath wrt the new devmapper convention

Due to the change on dev mapper linking/naming conventions, we should
avoid using realpath to chase to the final path. Also, we can now
rely on vhd-util itself to canonicalize the path.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #20 on xapi-project/sm