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/xen/Kconfig b/drivers/xen/Kconfig
new file mode 100644
index 0000000..6ad2a93
--- /dev/null
+++ b/drivers/xen/Kconfig
@@ -0,0 +1,10 @@
+config PVBLOCK
+	bool "Xen para-virtualized block device"
+	depends on DM
+	select BLK
+	select HAVE_BLOCK_DEVICE
+	help
+	  This driver implements the front-end of the Xen virtual
+	  block device driver. It communicates with a back-end driver
+	  in another domain which drives the actual block device.
+