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>