Add the ability to convert LVs of "mirror" segtype to "raid1" segtype.

Add the ability to convert LVs of "mirror" segtype to "raid1" segtype.

Example:
~> lvconvert --type raid1 vg/mirror_lv

Steps to convert "mirror" to "raid1"
1) Allocate a RAID metadata LV for each mirror image from the same PVs
   on which they are located.
2) Clear the metadata LVs.  This involves writing LVM metadata, so we don't
   change any aspects of the mirror LV before this so that the user can easily
   remove LVs from the failed convert attempt while retaining the original
   mirror.
3) Remove the mirror log, if it exists.
4) Add metadata LVs to mirror LV
5) Rename mirror sub-lvs (s/mimage/rimage/)
6) Change flags and segtype from mirror to raid1