One of my latest patches (d8db0f96) created support for setting
the limit for the maximum of opened files by qemu user. However,
since libvirtd keeps one FD opened per domain (well, for qemu at least)
it will likely hit this limit on huge scenarios.

This patch adds support for a systemd init service for libvirtd
and libvirt-guests. The libvirtd.service is *not* written to use
socket activation, since we want libvirtd to start on boot so it
can do guest auto-start.

The libvirt-guests.service is pretty lame, just exec'ing the
original init script for now. Ideally we would factor out the
functionality, into some shared tool.

Instead of

  ./configure --with-init-script=redhat

You can now do

  ./configure --with-init-script=systemd

Or better still:

  ./configure --with-init-script=systemd+redhat

We can also now support install of the upstart init script

* configure.ac: Add systemd, and systemd+redhat options to
  --with-init-script option
* daemon/Makefile.am: Install systemd services
* daemon/libvirtd.sysconf: Add note about unused env variable
  with systemd
* daemon/libvirtd.service.in: libvirtd systemd service unit
* libvirt.spec.in: Add scripts to installing systemd services
  and migrating from legacy init scripts
* tools/Makefile.am: Install systemd services
* tools/libvirt-guests.init.sh: Rename to tools/libvirt-guests.init.in
* tools/libvirt-guests.service.in: systemd service unit