common: _devmgt_add() to check if the device is back online

common: _devmgt_add() to check if the device is back online

btrfs/003 uses a method to remove the device as part of the test
case, and after the test completes the removed device is added
back to the system. However on certain system, albeit the slow
running system the device comes back a bit later, and so the
latter occurring sub-test with in the btrfs/003 fails.

This patch adds script to wait and test if the device is back online,
and thus report the same to to the full log.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>