Make diskless instances externally mirrored

This addresses Issue 237.

Mirroring no disk is a no-op. As such we can treat them like mirrored
instances, since the data they need (none) will be present on all nodes.

This is definitely enough to failover or migrate instances with a manual
target node. Further work might be needed for support in hail.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>