x86/sev: Prevent SECURE_AVIC_CONTROL MSR interception for Secure AVIC guests

x86/sev: Prevent SECURE_AVIC_CONTROL MSR interception for Secure AVIC guests

The SECURE_AVIC_CONTROL MSR holds the GPA of the guest APIC backing page and
bitfields to control enablement of Secure AVIC and whether the guest allows
NMIs to be injected by the hypervisor.

This MSR is populated by the guest and can be read by the guest to get the GPA
of the APIC backing page. The MSR can only be accessed in Secure AVIC mode.
Any attempt to access it when not in Secure AVIC mode results in #GP. So, the
hypervisor should not intercept it. A #VC exception will be generated
otherwise. If this occurs and Secure AVIC is enabled, terminate the guest
execution.

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/20250828113119.209135-1-Neeraj.Upadhyay@amd.com