-xen-attach is needed for pvh boot with qemu-xen

Currently the boot of a pvh guest using the qemu-xen device model
fails with the error
xen emulation not implemented (yet)
in the qemu-dm log file. This patch adds the missing -xen-attach
argument.

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>e
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ wei: ported to staging ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl_dm.c

@@ -1021,7 +1021,7 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
      */
     flexarray_append(dm_args, "-no-user-config");
 
-    if (b_info->type == LIBXL_DOMAIN_TYPE_PV) {
+    if (b_info->type != LIBXL_DOMAIN_TYPE_HVM) {
         flexarray_append(dm_args, "-xen-attach");
     }