[sheepdog] functional test 074 failure in master branch

[Ruoyu]

As titled. Only occurred in master. sheep will be crashed with the below 
error messages.

Aug 15 17:18:58  EMERG [block] cinfo_collection_work(712) PANIC: getting 
a snapshot of vdi state at epoch 2 failed
Aug 15 17:18:58  EMERG [block] crash_handler(268) sheep exits 
unexpectedly (Aborted).
Aug 15 17:18:58  EMERG [block] sd_backtrace(833) sheep.c:270: crash_handler
Aug 15 17:18:58  EMERG [block] sd_backtrace(847) 
/lib/x86_64-linux-gnu/libpthread.so.0(+0xfbaf) [0x7f99d44b0baf]
Aug 15 17:18:58  EMERG [block] sd_backtrace(847) 
/lib/x86_64-linux-gnu/libc.so.6(gsignal+0x36) [0x7f99d39e9f76]
Aug 15 17:18:58  EMERG [block] sd_backtrace(847) 
/lib/x86_64-linux-gnu/libc.so.6(abort+0x147) [0x7f99d39ed5e7]
Aug 15 17:18:58  EMERG [block] sd_backtrace(833) group.c:712: 
cinfo_collection_work
Aug 15 17:18:58  EMERG [block] sd_backtrace(833) work.c:348: worker_routine
Aug 15 17:18:58  EMERG [block] sd_backtrace(847) 
/lib/x86_64-linux-gnu/libpthread.so.0(+0x7f6d) [0x7f99d44a8f6d]
Aug 15 17:18:58  EMERG [block] sd_backtrace(847) 
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6c) [0x7f99d3aad9cc]