x86: consolidate microcode loading code

x86: consolidate microcode loading code

- memory was leaked on a CPU offline/online cycle (including S3)
- memory was leaked on AMD systems when microcode_update() ran a 2nd
  time with the same data that was used on the first run
- microcode never got restored on APs during S3 resume (or post-boot
  onlining of a CPU that was also online when microcode_update() first
  ran [in the event the prior microcode update got lost intermediately,
  which supposedly shouldn't happen]); this will still be the case when
  no other online CPU has an identical signature (which however is now
  consistent with bringing up such a CPU the very first time)
- resume was unimplemented in the AMD case
- there was a race between microcode_update_cpu() and
  microcode_resume_cpu()

This also moves vendor specific type declarations to the vendor source
file and sets the stage for boot time microcode loading (i.e. without
Dom0 involvement).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>