Add digprog mmio write method to avoid assert failure during
initialisation.

Reviewed-by: Li Qiang <liq3ea@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <20200811114133.672647-9-ppandit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>