xen: pvblock: Add initial support for para-virtualized block driver

Add initial infrastructure for Xen para-virtualized block device.
This includes compile-time configuration and the skeleton for
the future driver implementation.
Add new class UCLASS_PVBLOCK which is going to be a parent for
virtual block devices.
Add new interface type IF_TYPE_PVBLOCK.

Implement basic driver setup by reading XenStore configuration.

Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 119e412..613669c 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -136,6 +136,8 @@
 
 source "drivers/watchdog/Kconfig"
 
+source "drivers/xen/Kconfig"
+
 config PHYS_TO_BUS
 	bool "Custom physical to bus address mapping"
 	help