xen/sndif: Sync up with the canonical definition in Xen

This is the sync up with the canonical definition of the sound
protocol in Xen:

1. Protocol version was referenced in the protocol description,
   but missed its definition. Fixed by adding a constant
   for current protocol version.

2. Some of the request descriptions have "reserved" fields
   missed: fixed by adding corresponding entries.

3. Extend the size of the requests and responses to 64 octets.
   Bump protocol version to 2.

4. Add explicit back and front synchronization
   In order to provide explicit synchronization between backend and
   frontend the following changes are introduced in the protocol:
    - add new ring buffer for sending asynchronous events from
      backend to frontend to report number of bytes played by the
      frontend (XENSND_EVT_CUR_POS)
    - introduce trigger events for playback control: start/stop/pause/resume
    - add "req-" prefix to event-channel and ring-ref to unify naming
      of the Xen event channels for requests and events

5. Add explicit back and front parameter negotiation
   In order to provide explicit stream parameter negotiation between
   backend and frontend the following changes are introduced in the protocol:
   add XENSND_OP_HW_PARAM_QUERY request to read/update
   configuration space for the parameters given: request passes
   desired parameter's intervals/masks and the response to this request
   returns allowed min/max intervals/masks to be used.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>