vdpa: move to drivers/vdpa

We have both vhost and virtio drivers that depend on vdpa.
It's easier to locate it at a top level directory otherwise
we run into issues e.g. if vhost is built-in but virtio
is modular.  Let's just move it up a level.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 7a6d8b2..ac23d520 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -138,6 +138,8 @@
 
 source "drivers/virtio/Kconfig"
 
+source "drivers/vdpa/Kconfig"
+
 source "drivers/vhost/Kconfig"
 
 source "drivers/hv/Kconfig"