Disallow mirrored logs in cluster mirrors.

Disallow mirrored logs in cluster mirrors.

The cluster log daemon (cmirrord) is not multi-threaded and
can handle only one request at a time.  When a log is stacked
on top of a mirror (which itself contains a 'core' log), it
creates a situation that cannot be solved without threading.

When the top level mirror issues a "resume", the log daemon
attempts to read from the log device to retrieve the log
state.  However, the log is a mirror which, before issuing
the read, attempts to determine the 'sync' status of the
region of the mirror which is to be read.  This sync status
request cannot be completed by the daemon because it is
blocked on a read I/O to the very mirror requesting the
sync status.