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

Support for this is included in qemu and seabios from upstream git.

* src/qemu/qemu_capabilities.h (qemuCapsParseDeviceStr): New
prototype.
* src/qemu/qemu_capabilities.c (qemuCapsParsePCIDeviceStrs)
Rename and split...
(qemuCapsExtractDeviceStr, qemuCapsParseDeviceStr): ...to make it
easier to add and test device-specific checks.
(qemuCapsExtractVersionInfo): Update caller.
* tests/qemuhelptest.c (testHelpStrParsing): Also test parsing of
device-related flags.
(mymain): Update expected flags.
* tests/qemuhelpdata/qemu-0.12.1-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel60-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.12.3-device: New file.
* tests/qemuhelpdata/qemu-kvm-0.13.0-device: New file.

* src/qemu/qemu_capabilities.h (QEMUD_CMD_FLAG_MIGRATE_QEMU_FD):
New enum value.
* src/qemu/qemu_capabilities.c (qemuCapsComputeCmdFlags): Populate
flags according to qemu version.
* tests/qemuhelptest.c (mymain): Adjust test.

The qemu_conf.c code is doing three jobs, driver config file
loading, QEMU capabilities management and QEMU command line
management. Move the capabilities code into its own file

* src/qemu/qemu_capabilities.c, src/qemu/qemu_capabilities.h: New
  capabilities management code
* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Delete capabilities
  code
* src/qemu/qemu_conf.h: Adapt for API renames
* src/Makefile.am: add src/qemu/qemu_capabilities.c