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: Oleksandr Andrushchenko
Reviewed-by: Juergen Gross
Signed-off-by: Takashi Iwai