qemu: allow pulseaudio to be the default

We're seeing various issues with the SDL audio backend and want to
switch to the pulseaudio backend. See e.g.

  https://bugzilla.redhat.com/495964
  https://bugzilla.redhat.com/519540
  https://bugzilla.redhat.com/496627



The pulseaudio backend seems to work well, so we should allow it to be
selected as the default.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: malc <av1474@comtv.ru>