net/9p: Use the tag name in the config space for identifying mount point

This patch use the tag name in the config space to identify the
mount device. The the virtio device name depend on the enumeration
order of the device and may not remain the same across multiple boots
So we use the tag name which is set via qemu option to uniquely identify
the mount device

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

include/linux/virtio_9p.h

@@ -5,4 +5,16 @@
 #include <linux/virtio_ids.h>
 #include <linux/virtio_config.h>
 
+/* The feature bitmap for virtio 9P */
+
+/* The mount point is specified in a config variable */
+#define VIRTIO_9P_MOUNT_TAG 0
+
+struct virtio_9p_config {
+	/* length of the tag name */
+	__u16 tag_len;
+	/* non-NULL terminated tag name */
+	__u8 tag[0];
+} __attribute__((packed));
+
 #endif /* _LINUX_VIRTIO_9P_H */