WARN if the save state size for a valid XCR0-managed sub-leaf is zero,
which would indicate a KVM or CPU bug.  Add a comment to explain why KVM
WARNs so the reader doesn't have to tease out the relevant bits from
Intel's SDM and KVM's XCR0/XSS code.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>