audio: check for pulseaudio daemon pidfile

Check whenever the pulseaudio daemon pidfile is present before trying to
initialize the pulseaudio backend.  Just return NULL if that is not the
case, so qemu will check the next backend in line.

In case the user explicitly configured a non-default pulseaudio server
skip the check.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20190124112055.547-5-kraxel@redhat.com