libxl: fix guest networking on NetBSD

libxl: fix guest networking on NetBSD

As previously reported when I start guests with xl then the
guest network does not work because the qemu-ifup script
no longer runs.

NetBSD doesn't have something like udev. Changing xm/xend,
libxl and xenbackendd to make everything behave the same way
is a lot more intrusive than enabling it for NetBSD again.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>