Merge pull request #388 from euanh/build-improvements

Remove ocaml-nbd's dependency on opam, and fix fallout.
mock plugins: Enable tmpfs and disable package_state

These changes make chroot operations faster.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #389 from euanh/master

mock plugins: Enable tmpfs and disable package_state
vhd-tool: Update to 0.7.4

Signed-off-by: Euan Harris <euan.harris@citrix.com>