Currently all host audio backends are disabled if a VM is using VNC, in
favor of the QEMU VNC audio extension. Unfortunately no released VNC
client supports this extension, so users have no way of getting audio
to work if using VNC.

Add a new config option in qemu.conf which allows changing libvirt's
behavior, but keep the default intact.

v2: Fix doc typos, change name to vnc_allow_host_audio