sheepdog: check return values of qemu_co_recv/send correctly

If qemu_co_recv/send doesn't return the specified length, it means
that an error happened.

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>
