sndif: Add explicit back and front synchronization

sndif: 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

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: Takashi Iwai <tiwai@suse.de>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Juergen Gross <jgross@suse.com>