qemu: Generate cmd line for pipewire audio backend

qemu: Generate cmd line for pipewire audio backend

This is mostly straightforward, except for a teensy-weensy
detail: usually, there's no system wide daemon running, no system
wide available socket that anybody could connect to. PipeWire
uses a per user daemon approach instead. But this in turn means,
that the socket location floats between various locations and is
derived from various environment variables (just like the actual
socket name) and thus we must pass the variables to QEMU.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/560
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>