kvm: vmx: Reinstate support for CPUs without virtual NMI

This is more or less a revert of commit 2c82878b ("KVM: VMX: require
virtual NMI support", 2017-03-27); it turns out that Core 2 Duo machines
only had virtual NMIs in some SKUs.

The revert is not trivial because in the meanwhile there have been several
fixes to nested NMI injection.  Therefore, the entire vNMI state is moved
to struct loaded_vmcs.

Another change compared to before the patch is a simplification here:

       if (unlikely(!cpu_has_virtual_nmis() && vmx->soft_vnmi_blocked &&
           !(is_guest_mode(vcpu) && nested_cpu_has_virtual_nmis(
                                       get_vmcs12(vcpu))))) {

The final condition here is always true (because nested_cpu_has_virtual_nmis
is always false) and is removed.

Fixes: 2c82878b
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1490803


Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>