xen: pvblock: Read XenStore configuration and initialize

Read essential virtual block device configuration data from XenStore,
initialize front ring and event channel.
Update block device description with actual block size.

Use code for XenStore from mini-os.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>