x86/microcode: Check whether the microcode is correct

x86/microcode: Check whether the microcode is correct

We do the microcode code update in two steps - the presmp:
'microcode_presmp_init' and when CPUs are brought up: 'microcode_init'.
The earlier performs the microcode update on the BSP - but
unfortunately it does not check whether the update failed. Which means
that we might try later to update a incorrect payload on the rest of
CPUs.

This patch handles this odd situation.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>