vhd-format: fix remove for 0.6.0 and 0.6.4

Fixes [ocaml/opam-repository#2701]

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #2702 from djs55/vhd-format-uninstall

vhd-format: fix remove for 0.6.0 and 0.6.4