In QEMU, the card itself is a PCI device, but it requires a codec
(either -device hda-output or -device hda-duplex) to actually output
sound. Specifying <sound model='ich6'/> gives us -device intel-hda
-device hda-duplex I think it's important that a simple <sound model='ich6'/>
sets up a useful codec, to have consistent behavior with all other sound cards.

This is basically Dan's proposal of

    <sound model='ich6'>
        <codec type='output' slot='0'/>
        <codec type='duplex' slot='3'/>
    </sound>

without the codec bits implemented.

The important thing is to keep a consistent API here, we don't want some
<sound> devs require tweaking codecs but not others. Steps I see to
accomplishing this:

    - every <sound> device has a <codec type='default'/> (unless codecs are
        manually specified)
    - <codec type='none'/> is required to specify 'no codecs'
    - new audio settings like mic=on|off could then be exposed in
        <sound> or <codec> in a consistent manner for all sound models

v2:
    Use model='ich6'

v3:
    Use feature detection, from eblake
    Set codec id, bus, and cad values

v4:
    intel-hda isn't supported if -device isn't available

v5:
    Comment spelling fixes