Fix for bug 734252 - problem up converting striped mirror after image failure

Fix for bug 734252 - problem up converting striped mirror after image failure

lv_mirror_count was not able to handle mirrors of stripes properly.  When a
failed device is removed, the MIRRORED status flag is removed from the LV
conditionally based on the results of lv_mirror_count.  However, lv_mirror_count
trusted the MIRRORED flag - thinking any such LV must be mirrored.  It would
happily assign first_seg(lv)->area_count as the number of mirrors, but when
a mirrored striped LV was reduced to a simple striped LV area_count would be
the number of /stripes/ not the number of /mirrors/.  A result higher than 1
would be returned from lv_mirror_count, the MIRRORED flag would not be cleared,
and the LV would fail to be up-converted properly in lvconvert_mirrors_aux
because of it.