tools: add basic libxendevicemodel with _open and _close.
This library is intended to be a stable interface which device models
(specifically QEMU upstream, it probably isn't worth porting
qemu-traditional over) can rely on in order to avoid a link time
dependency on libxenctrl (which implies needing to rebuild QEMU when
Xen is updated to a new release, which is a pain for distros)
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>