This resolves one of the issues in:

   https://bugzilla.redhat.com/show_bug.cgi?id=1003983

This device is identical to qemu's "intel-hda" device (known as "ich6"
in libvirt), but has a different PCI device ID (which matches the ID
of the hda audio built into the ich9 chipset, of course). It's not
supported in earlier versions of qemu, so it requires a capability
bit.