Fix for bug 733114.

Fix for bug 733114.

When an image is split from a 2-way mirror, the original mirror is converted to
a linear device.  To do this, the top "layer" must be removed.  The segments
are transferred from the sub-lv to the top-level LV and the link is severed.
The former sub-lv - having its segments transferred - now contains a temporary
error target.

When the original LV is resumed, the old sub-lv that now contains an error
segment is activated and scanned.  This is what causes the I/O error messages.
There are three ways to fix this problem:

1) Do not set the sub-lv which contains the error target as "visible" before
suspending the original LV.  This way, when the original is resumed, the sub-lv
device node is not created and it is not scanned - avoiding the error messages.
 The problem with this approach is that if the machine crashes after the
resume, it leaves the *hidden* LV in place and the user has a more difficult
time noticing that it needs to be cleaned up.  Thus, this type of processing is
frowned upon.

2) Do like _remove_mirror_images does and suspend the original, then suspend
the sub-lv (the error target), then resume the sub-lv, and finally resume the
original LV.  This seems like extra pointless operations to me, but it does not
produce the error message (although, I'm not sure why) and it allows us to
leave the visible flag in place.

3) Flag the sub-lv (error target) with a "do not scan" flag.  This seems like
the cleanest approach, but I have been unable to find the method for doing
this.  LVs get tagged in such a way by _get_udev_flags, but in this case the
resume of the original LV also resumes the error target LV without running it
through _get_udev_flags (likely because they are no longer linked).  Could
there be something wrong in resume_lv?

Option #2 was chosen to fix this bug, but it seems like more of a workaround
for now.