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/cmd/Kconfig b/cmd/Kconfig
index 23d7e27..9ad511a 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1370,6 +1370,13 @@
 	help
 	  USB mass storage support
 
+config CMD_PVBLOCK
+	bool "Xen para-virtualized block device"
+	depends on XEN
+	select PVBLOCK
+	help
+	  Xen para-virtualized block device support
+
 config CMD_VIRTIO
 	bool "virtio"
 	depends on VIRTIO