nbd-lwt-unix: add lower bound on nbd package to ensure compatibility
nbd-lwt-unix: add lower bound on nbd package to ensure compatibility

This is to ensure that we will compile nbd-lwt-unix with a compatible
version of the nbd library. Nbd 4.0.0 has introduced some new values
that this version of nbd-lwt-unix relies on, it does not work with nbd
3.X.

Signed-off-by: Gabor Igloi <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

xapi-project/xs-opam,lindig/xs-opam