loader: add support for passing verity hash to xen kernels

This only supports DomU Linux bzImage, ignoring bare ELF images and
Dom0 Xen+Linux but those cases are not applicable to us on CoreOS.

grub-core/loader/i386/xen.c

@@ -416,7 +416,9 @@ grub_cmd_xen (grub_command_t cmd __attribute__ ((unused)),
   if (!file)
     return grub_errno;
 
-  elf = grub_xen_file (file);
+  elf = grub_xen_file_and_cmdline (file,
+		                   (char *) next_start.cmd_line,
+		                   sizeof (next_start.cmd_line) - 1);
   if (!elf)
     goto fail;