kvm: vmx: Introduce VMCS12_MAX_FIELD_INDEX

This is the highest index value used in any supported VMCS12 field
encoding. It is used to populate the IA32_VMX_VMCS_ENUM MSR.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>