CA-113322: let vhd-util-modify canonicalize the parent path passed in.

We shouldn't rely on the caller to always pass in sanctified paths.With
the new dev mapper linking/mapping convention, the realpath function
applied on the caller (SM) won't give the kind of path vhd-util-modify
expects (I'll create another patchset in SM to address this).

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

GitHub: closes #46 on xapi-project/blktap