hw/xen: use correct default protocol for xen-block on x86

Even on x86_64 the default protocol is the x86-32 one if the guest doesn't
specifically ask for x86-64.

Cc: qemu-stable@nongnu.org
Fixes: b6af892 ("xen: add implementations of xen-block connect and disconnect functions...")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
(cherry picked from commit a1c1082)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/block/xen-block.c

@@ -115,9 +115,13 @@ static void xen_block_connect(XenDevice *xendev, Error **errp)
         return;
     }
 
-    if (xen_device_frontend_scanf(xendev, "protocol", "%ms",
-                                  &str) != 1) {
-        protocol = BLKIF_PROTOCOL_NATIVE;
+    if (xen_device_frontend_scanf(xendev, "protocol", "%ms", &str) != 1) {
+        /* x86 defaults to the 32-bit protocol even for 64-bit guests. */
+        if (object_dynamic_cast(OBJECT(qdev_get_machine()), "x86-machine")) {
+            protocol = BLKIF_PROTOCOL_X86_32;
+        } else {
+            protocol = BLKIF_PROTOCOL_NATIVE;
+        }
     } else {
         if (strcmp(str, XEN_IO_PROTO_ABI_X86_32) == 0) {
             protocol = BLKIF_PROTOCOL_X86_32;