sheepdog: try to reconnect to sheepdog after network error

sheepdog: try to reconnect to sheepdog after network error

This introduces a failed request queue and links all the inflight
requests to the list after network error happens.  After QEMU
reconnects to the sheepdog server successfully, the sheepdog block
driver will retry all the requests in the failed queue.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Tested-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>