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>