libxc: NetBSD: implement xc_evtchn_bind_unbound_port.

libxc: NetBSD: implement xc_evtchn_bind_unbound_port.

Doesn't actually appear to be used anywhere but is defined for other
OSes.

The NetBSD evtchn.h contains comments "Return allocated port" for
several ioctls which currently return the allocated port as a member
of the argument structure and not as the ioctl return value (I think
this is a cut and paste error).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Christoph Egger <Christoph.Egger@amd.com>
committer: Stefano Stabellini <stefano.stabellini@eu.citrix.com>