KVM: SVM: Add Virtual GIF feature definition

Add a new cpufeature definition for Virtual GIF.

Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>