Implement essential initialization of the sound driver:
  - introduce required data structures
  - handle driver registration
  - handle sound card registration
  - register sound driver on backend connection
  - remove sound driver on backend disconnect

Initialize virtual sound card with streams according to the
Xen store configuration.

Implement ALSA driver operations including:
  - manage frontend/backend shared buffers
  - manage Xen bus event channel states

Implement requests from front to back for ALSA
PCM operations.
  - report ALSA period elapsed event: handle XENSND_EVT_CUR_POS
    notifications from the backend when stream position advances
    during playback/capture. The event carries a value of how
    many octets were played/captured at the time of the event.
  - implement explicit stream parameter negotiation between
    backend and frontend: handle XENSND_OP_HW_PARAM_QUERY request
    to read/update configuration space for the parameter given:
    request passes desired parameter interval and the response to
    this request returns min/max interval for the parameter to be used.
Signed-off-by: NOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Implement shared buffer handling according to the
para-virtualized sound device protocol at xen/interface/io/sndif.h:
  - manage buffer memory
  - handle granted references
  - handle page directories

[ Fixed missing linux/kernel.h inclusion -- tiwai ]
Signed-off-by: NOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Handle Xen event channels:
  - create for all configured streams and publish
    corresponding ring references and event channels in Xen store,
    so backend can connect
  - implement event channels interrupt handlers
  - create and destroy event channels with respect to Xen bus state
Signed-off-by: NOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Read configuration values from Xen store according
to xen/interface/io/sndif.h protocol:
  - introduce configuration structures for different
    components, e.g. sound card, device, stream
  - read PCM HW parameters, e.g rate, format etc.
  - detect stream type (capture/playback)
  - read device and card parameters
Signed-off-by: NOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Introduce skeleton of the para-virtualized Xen sound
frontend driver.

Initial handling for Xen bus states: implement
Xen bus state machine for the frontend driver according to
the state diagram and recovery flow from sound para-virtualized
protocol: xen/interface/io/sndif.h.
Signed-off-by: NOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>