ocaml-qmp: Add 'change' command

The change command is used both to change removable media and reconfigure
VNC.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
build: messages depends on unix

After upgrading our OUnit package to 2.0.0, messages stopped
building without an explicit dependency on unix.

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