KVM: selftests: nVMX: Properly deal with 'hv_clean_fields'

Instead of just resetting 'hv_clean_fields' to 0 on every enlightened
vmresume, do the expected cleaning of the corresponding bit on enlightened
vmwrite. Avoid direct access to 'current_evmcs' from evmcs_test to support
the change.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220203104620.277031-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>