sheep: call stat_request_end() only when stat_request_begin() was called

When sheepdog is not formated yet, sheep can call stat_request_end()
without calling stat_request_begin() and it leads to a segfault.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
tests/functional: add test for live snapshot under heavy workload

This reproduces the bug reported by Valerio in

  http://lists.wpkg.org/pipermail/sheepdog-users/2013-August/001209.html

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: fix check of snapshot vdi

This fixes a bug introduced by the commit 996751b.

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