sheepdog: use non-blocking fd in coroutine context

Using a blocking socket in the coroutine context reduces the chance of
switching to other work.  This patch makes the sheepdog driver use a
non-blocking fd always.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>