sheep: add remove_vdisks to remove virtual disks

We don't need to iterate over all the vnodes to remove a disk.

This prepares for the next patch.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
tests/unit: use rbtree

This fixes unittests which are broken recent commits.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>