x86/sev: Enable NMI support for Secure AVIC

x86/sev: Enable NMI support for Secure AVIC

Now that support to send NMI IPI and support to inject NMI from the hypervisor
has been added, set V_NMI_ENABLE in the VINTR_CTRL field of the VMSA to enable
NMI for Secure AVIC guests.

  [ bp: Zap useless brackets. ]

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/20250828111315.208959-1-Neeraj.Upadhyay@amd.com