tools: xenbackendd for NetBSD

tools: xenbackendd for NetBSD

Attached patch introduces xenbackendd. It is used on NetBSD
to launch the hotplug scripts. Later xenbackendd can be improved to
also launch qemu-dm as child process and will notice when qemu-dm
crashes.

The changes the patch makes:
- rename hotplug scripts as xenbackendd expects them
  (current names were taken from pkgsrc)
- install hotplug scripts as executable scripts
- introduce xenbackendd
- build/install/launch on NetBSD only

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>