On my machine, a guest fails to boot if it has a sound card, but not
graphical device/display is configured, because pulseaudio fails to
initialize since it can't access $HOME.

A workaround is removing the audio device, however on ARM boards there
isn't any option to do that, so -nographic always fails.

Set QEMU_AUDIO_DRV=none if no <graphics> are configured. Unfortunately
this has massive test suite fallout.

Add a qemu.conf parameter nographics_allow_host_audio, that if enabled
will pass through QEMU_AUDIO_DRV from sysconfig (similar to
vnc_allow_host_audio)

Qemu's implementation of virtio RNG supports rate limiting of the
entropy used. This patch exposes the option to tune this functionality.

This patch is based on qemu commit 904d6f588063fb5ad2b61998acdf1e73fb4

The rate limiting is exported in the XML as:
<devices>
  ...
  <rng model='virtio'>
    <rate bytes='123' period='1234'/>
    <backend model='random'/>
  </rng>
  ...

There is some controversy[1] on the qemu list on whether qemu should
have ever allowed arbitrary file name passthrough, or whether it
should be restricted to JUST /dev/random and /dev/hwrng.  It is
always easier to add support for additional filenames than it is
to remove support for something once released, so this patch
restricts libvirt 1.0.3 (where the virtio-random backend was first
supported) to just the two uncontroversial names, letting us defer
to a later date any decision on whether supporting arbitrary files
makes sense. Additionally, since qemu 1.4 does NOT support
/dev/fdset/nnn fd passthrough for the backend, limiting to just
two known names means that we don't get tempted to try fd
passthrough where it won't work.

[1]https://lists.gnu.org/archive/html/qemu-devel/2013-03/threads.html#00023

* src/conf/domain_conf.c (virDomainRNGDefParseXML): Only allow
/dev/random and /dev/hwrng.
* docs/schemas/domaincommon.rng: Flag invalid files.
* docs/formatdomain.html.in (elementsRng): Document this.
* tests/qemuxml2argvdata/qemuxml2argv-virtio-rng-random.args:
Update test to match.
* tests/qemuxml2argvdata/qemuxml2argv-virtio-rng-random.xml:
Likewise.

Users may want to specify XML entities in paths to devices. Ensure they
are parsed and used properly.

Adds XML parsing and qemu commandline tests for the VirtIO RNG device
support.