Fix: agent: prevent status action from hanging on list command if any of the devices is silently blocked

If any of the configured SBD devices is silently blocked without any
explicit I/O error from kernel, status action will get stuck on list
command which will be hanging on exit_aio() and become D state.

With this commit, sbd fence agent asynchronously calls list command
individually for the devices and won't wait for any devices that
actually have been already reported failed, so that it prevents hanging
under such a situation.