Finish fix for bug 607347: failing both redundant mirror log legs...

A previous check-in added logic to handle the case where both images
of a mirrored log failed.  It solved the problem by simply removing
the log entirely - leaving the parent mirror with a 'core' log.  This
worked for most cases.  However, if there was a small delay between
the failures of the two mirrored log devices, the mirror would hang,
LVM would hang, and no additional LVM commands could be issued.

When the first leg of the log fails, it signals the need for repair.
Before 'lvconvert --repair' is run by dmeventd, the second leg fails.
'lvconvert' would see both devices as failed and try to remove the
log entirely.  When it came time to suspend the parent mirror to
update the configuration, the suspend would hang because it couldn't
get any I/O through the mirrored log, which was plugged waiting for
corrective action.  The solution is to replace the log with an error
target to clear any pending writes before removing it.  This allows
the parent mirror to suspend and make the proper changes.

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