guid: Unify GUID types

There are 3 implementations of a GUID in GRUB. Replace them with
a common one, placed in types.h.

It uses the "packed" flavor of the GUID structs, the alignment attribute
is dropped, since it is not required.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/loader/i386/xnu.c

@@ -48,7 +48,7 @@ grub_uint32_t grub_xnu_entry_point, grub_xnu_arg1, grub_xnu_stack;
 /* Aliases set for some tables. */
 struct tbl_alias
 {
-  grub_efi_guid_t guid;
+  grub_guid_t guid;
   const char *name;
 };
 
@@ -694,7 +694,7 @@ grub_cpu_xnu_fill_devicetree (grub_uint64_t *fsbfreq_out)
     {
       void *ptr;
       struct grub_xnu_devtree_key *curkey;
-      grub_efi_packed_guid_t guid;
+      grub_guid_t guid;
       char guidbuf[64];
 
       /* Retrieve current key. */