x86/apic: Enable Secure AVIC in the control MSR

With all the pieces in place now, enable Secure AVIC in the Secure AVIC
Control MSR. Any access to x2APIC MSRs are emulated by the hypervisor
before Secure AVIC is enabled in the control MSR.  Post Secure AVIC
enablement, all x2APIC MSR accesses (whether accelerated by AVIC
hardware or trapped as a #VC exception) operate on the vCPU's APIC
backing page.

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Link: https://lore.kernel.org/20250828112126.209028-1-Neeraj.Upadhyay@amd.com