Fix reversal of LV list before performing a split mirror.

Fix reversal of LV list before performing a split mirror.

When splitting off mirror images from a mirror, we always take
LVs from the end of a list.  For example, if the mirror sub-devices
are lv_mimage_[012], we should select lv_mimage_2 if splitting off
one image.  However, lv_mimage_0 was being selected instead.

The problem came from calling '_move_removable_mimages_to_end'
when it was unnecessary to do so.  When the user /does/ specify
specific devices to be removed, this function properly moved the
appropriate LVs to the end of the list for extraction.  However,
if the user /doesn't/ give any specific PVs, the function should
do nothing.  '_move_removable_mimages_to_end' was keying off of
whether 'removable_pvs' was NULL or not and this value was
improperly being populated with the set of all available PVs.
This was causing '_move_removable_mimages_to_end' to completely
reverse the list, which in turn caused us to extract the
hithertofore front-of-the-list LVs.