Update libvirt to 2.4

Update libvirt to 2.4

This patch update libvirt to version 2.4 for changelog see:
https://libvirt.org/news-2016.html

This patch also add some new configure switches:

* We build now only for qemu/kvm. The build for VMware and
  phyp are now disabled.

* And libvirt is now explicitly built without SELinux.
  This change was necessary because of the fact that
  a build dependency of libvirt required SELinux
  and so libvirt is also linked against SELinux
  because it is available in the build environment.

  Until we get completely rid of SELinux we should use
  these switches.
  So we have a libvirt that does not require SELinux
  otherwise, we would have to release a version when we
  dropped SELinux completely, which would be stupid.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>