Fix issue preventing cluster mirror creation.

Fix issue preventing cluster mirror creation.

Mirrors used to be created by first creating a linear device and then adding
the other images plus the log.  Now mirrors are created by creating all the
images in one go and then adding the log separately.  The new way ran into
the condition that cluster mirrors cannot change the log type (in the case
of creation, from core -> disk) while the mirror is not active.  (It isn't
active because it is in the process of being created.)  The reason this
condition is in place is because a remote node may have the mirror active, and
we don't want to alter the log underneath it.

What we really needed was a way of checking if the mirror was active remotely
but not locally, and in that case do not allow a change of the log.  I've added
this check, and cluster mirrors can now be created again.