This patch adds the capability to split off a mirror legs.

This patch adds the capability to split off a mirror legs.
It is pretty much the same as reducing the number of
mirror legs, but we just don't delete them afterwards.

The following command line interface is enforced:
  prompt> lvconvert --splitmirror <n> -n <name> <VG>/<LV>
where 'n' is the number of images to split off, and
where 'name' is the name of the newly split off logical volume.

If more than one leg is split off, a new mirror will be the
result.  The newly split off mirror will have a 'core' log.
Example:
[root@bp-01 LVM2]# !lvs
lvs -a -o name,copy_percent,devices
  LV            Copy%  Devices
  lv            100.00 lv_mimage_0(0),lv_mimage_1(0),lv_mimage_2(0),lv_mimage_3(0)
  [lv_mimage_0]        /dev/sdb1(0)
  [lv_mimage_1]        /dev/sdc1(0)
  [lv_mimage_2]        /dev/sdd1(0)
  [lv_mimage_3]        /dev/sde1(0)
  [lv_mlog]            /dev/sdi1(0)
[root@bp-01 LVM2]# lvconvert --splitmirrors 2 --name split vg/lv /dev/sd[ce]1
  Logical volume lv converted.
[root@bp-01 LVM2]# !lvs
lvs -a -o name,copy_percent,devices
  LV               Copy%  Devices
  lv               100.00 lv_mimage_0(0),lv_mimage_2(0)
  [lv_mimage_0]           /dev/sdb1(0)
  [lv_mimage_2]           /dev/sdd1(0)
  [lv_mlog]               /dev/sdi1(0)
  split            100.00 split_mimage_0(0),split_mimage_1(0)
  [split_mimage_0]        /dev/sde1(0)
  [split_mimage_1]        /dev/sdc1(0)

It can be seen that '--splitmirror <n>' is exactly the same
as '--mirrors -<n>' (note the minus sign), except there is the
additional notion to keep the image being detached from the
mirror instead of just throwing it away.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>