KVM: x86: nSVM: expose clean bit support to the guest

KVM already honours few clean bits thus it makes sense
to let the nested guest know about it.

Note that KVM also doesn't check if the hardware supports
clean bits, and therefore nested KVM was
already setting clean bits and L0 KVM
was already honouring them.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220207155447.840194-6-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>