vhd: rename 'openfile' to 'openchain'; add 'openfile'

openchain: opens every vhd file on a path to the root or
  fails trying

openfile: opens only the named vhd file

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #19 from djs55/openchain

vhd: rename 'openfile' to 'openchain'; add 'openfile'