Enhance mirror operations for DRBD8

Currently, the mirror operations (add and remove children) test against
the instance's attributes. This patch changes the check tests to work
against the actual status of the device (i.e. live data) which is more
realistic.

The changes are:
  - allow add children if the device doesn't have local storage (even if
    we believe it has)
  - early return from remove children if the device is already without
    local storage

Reviewed-by: imsnah