KVM: VMX: Rename XSAVES control to follow KVM's preferred "ENABLE_XYZ"

Rename the XSAVES secondary execution control to follow KVM's preferred
style so that XSAVES related logic can use common macros that depend on
KVM's preferred style.

No functional change intended.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20230815203653.519297-6-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>