ALSA: xen-front: Read sound driver configuration from Xen store

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: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>