sheep: propagate deletion information via SD_OP_GET_VDI_COPIES

The commit f68feab7edc0de introduced significant optimization for dog
vdi list. But the deleted vid bitmap cannot be copied to newly joining
sheep. This patch let new sheeps copy the information via
SD_OP_GET_VDI_COPIES during joining process.

Below is the evaluation result by Saeki-san:

1. create 1000 snapshot
2. delete  999 snapshot

3. add new node at -p 7003
  # sheep -p 7003 -z 3 /var/lib/sheepdog/data3
( wait for complete recovery )

4. test vdi list to original node
  # dog vdi list -p 7000 -T
===> [test-case-1]
5. test vdi list to additional node
  # dog vdi list -p 7003 -T
===> [test-case-2]

before patch applied
[test-case-1]  Elapsed time: 0.083 seconds
[test-case-2] Elapsed time: 1.134 seconds

after patch applied
[test-case-1]  Elapsed time: 0.070seconds
[test-case-2]  Elapsed time: 0.167 seconds

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