verifiers: File type for fine-grained signature-verification controlling

Let's provide file type info to the I/O layer. This way verifiers
framework and its users will be able to differentiate files and verify
only required ones.

This is preparatory patch.

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>

grub-core/loader/i386/xen_file.c

@@ -78,7 +78,7 @@ grub_xen_file (grub_file_t file)
      Trim it.  */
   if (grub_memcmp (magic, XZ_MAGIC, sizeof (XZ_MAGIC) - 1) == 0)
     payload_length -= 4;
-  off_file = grub_file_offset_open (file, payload_offset,
+  off_file = grub_file_offset_open (file, GRUB_FILE_TYPE_LINUX_KERNEL, payload_offset,
 				    payload_length);
   if (!off_file)
     goto fail;