xen: import ring.h from xen

Do not use the ring.h header installed on the system. Instead, import
the header into the QEMU codebase. This avoids problems when QEMU is
built against a Xen version too old to provide all the ring macros.
Signed-off-by: NStefano Stabellini <stefano@aporeto.com>
Reviewed-by: NGreg Kurz <groug@kaod.org>
CC: anthony.perard@citrix.com
CC: jgross@suse.com

hw/block/xen_blkif.h

 #ifndef XEN_BLKIF_H
 #define XEN_BLKIF_H
 
-#include <xen/io/ring.h>
+#include "hw/xen/io/ring.h"
 #include <xen/io/blkif.h>
 #include <xen/io/protocols.h>
 

hw/usb/xen-usb.c

@@ -33,7 +33,7 @@
 #include "qapi/qmp/qint.h"
 #include "qapi/qmp/qstring.h"
 
-#include <xen/io/ring.h>
+#include "hw/xen/io/ring.h"
 #include <xen/io/usbif.h>
 
 /*