sheep/dog: introduce new bitmap for delete vdi

For day-to-day backup, repeatedly create snapshot and delete snapshot.
but inode file remains even after delete snapshot.
"dog vdi list" command would read all inode files, even if vdi/snapshot was deleted.
this behavior is inefficient to get current VDI list.

This patch manages the deleted VDI in a bit map.
And, referring VDI bit map when you get VDI list, to open only the necessary files.

Assessment procedure of this patch are below

1. create VDI 1000 times
2. clearing file cache
3. execute "dog vdi list -T"
===> execution time assessment [case1]
4. create and delete snapshot 4000 times
5. clearing file cache
6. execute "dog vdi list -T"
===> execution time assessment [case2]
7. create and delete snapshot 5000 times
8. clearing file cache
9. execute "dog vdi list -T"
===> execution time assessment [case3]

results are follow.

* Current Sheepdog
[case1] Elapsed time: 0.963 seconds
[case2] Elapsed time: 6.522 seconds
[case3] Elapsed time: 23.376 seconds

* Patch applied Sheepdog
[case1] Elapsed time: 0.714 seconds
[case2] Elapsed time: 0.979 seconds
[case3] Elapsed time: 0.921 seconds

Signed-off-by: Masaki Saeki <saeki.masaki@po.ntts.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>