xen/microcode: Use dummy microcode_ops for non initial domain guest

xen/microcode: Use dummy microcode_ops for non initial domain guest

Orabug: 19053626

Currently non initial domain guest use Intel or AMD specific ops.
This will also slow up the startup on heavily overcommited guests (say 256VCPUs
on 20 PCPU), as there are many read and write to x86 MSR registers which will
trap to xen during microcode update. Finally it will fail and report errors.

A dummy ops could fix that and also make udevd silent (bug18379824)
by augmenting the commit c18a317f6892536851e5852b6aaa4ef42cbc11a2
"xen/microcode: Only load under initial domain." which fell short of its
intended fix.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Guangyu Sun <guangyu.sun@oracle.com>
(cherry picked from commit 62b84234f23c1020c690d162b7d8250042425e1e)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Conflicts:
	arch/x86/kernel/microcode_core.c