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

When using VNC for graphics + keyboard + mouse, we shouldn't
then use the host OS for audio. Audio should go back over
VNC.

When using SDL for graphics, we should use the host OS for
audio since that's where the display is. We need to allow
certain QEMU env variables to be passed through to guest
too to allow choice of QEMU audio backend.

* qemud/libvirtd.sysconf: Mention QEMU/SDL audio env vars
* src/qemu_conf.c: Passthrough QEMU/SDL audio env for SDL display,
  disable host audio for VNC display

* qemud/: Rename to daemon/
* Makefile.am, configure.in, src/Makefile.am, src/remote_internal.c,
  tests/Makefile.am, tests/eventtest.c: s/qemud/daemon/ where needed