x86/sev: Initialize VGIF for secondary vCPUs for Secure AVIC

x86/sev: Initialize VGIF for secondary vCPUs for Secure AVIC

Virtual GIF (VGIF) provides masking capability for when virtual interrupts
(virtual maskable interrupts, virtual NMIs) can be taken by the guest vCPU.

The Secure AVIC hardware reads VGIF state from the vCPU's VMSA. So, set VGIF for
secondary CPUs (the configuration for the boot CPU is done by the hypervisor),
to unmask delivery of virtual interrupts  to the vCPU.

Signed-off-by: Kishon Vijay Abraham I <kvijayab@amd.com>
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/20250828111141.208920-1-Neeraj.Upadhyay@amd.com