conf: Introduce pipewire audio backend

conf: Introduce pipewire audio backend

QEMU gained support for PipeWire audio backend (see QEMU commit
of v8.0.0-403-gc2d3d1c294). Its configuration knobs are basically
the same as pulseaudio's, except for PA's server name. Therefore,
a lot of code is copied over from pulseadio and fixed by
s/Pulse/Pipewire/ or s/pulseaudio/pipewire/.

There's one ley difference to PA though: pipewire daemon is
usually on per user basis (just like our qemu:///session).
Therefore, introduce this 'runtimeDir' attribute, which allows
specifying path to pipewire daemon socket (useful for
qemu:///system for instance).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>