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/include/blk.h b/include/blk.h
index abcd4be..9ee10fb 100644
--- a/include/blk.h
+++ b/include/blk.h
@@ -33,6 +33,7 @@
 	IF_TYPE_HOST,
 	IF_TYPE_NVME,
 	IF_TYPE_EFI,
+	IF_TYPE_PVBLOCK,
 	IF_TYPE_VIRTIO,
 
 	IF_TYPE_COUNT,			/* Number of interface types */