xen/9pfs: build 9pfs Xen transport driver

This patch adds a Kconfig option and Makefile support for building the
9pfs Xen driver.

CC: groug@kaod.org
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
CC: Eric Van Hensbergen <ericvh@gmail.com>
CC: Ron Minnich <rminnich@sandia.gov>
CC: Latchesar Ionkov <lucho@ionkov.net>
CC: v9fs-developer@lists.sourceforge.net

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

net/9p/Kconfig

@@ -22,6 +22,14 @@ config NET_9P_VIRTIO
 	  This builds support for a transports between
 	  guest partitions and a host partition.
 
+config NET_9P_XEN
+	depends on XEN
+	tristate "9P Xen Transport"
+	help
+	  This builds support for a transport for 9pfs between
+	  two Xen domains.
+
+
 config NET_9P_RDMA
 	depends on INET && INFINIBAND && INFINIBAND_ADDR_TRANS
 	tristate "9P RDMA Transport (Experimental)"