No need to answer to simcall when host is off.

No need to answer to simcall when host is off.

These changes should fix tests platform-failures where some processes were
duplicated in process_to_run[], leading to runtime errors with boost and raw
contexts.

The error could be seen with a "xbt_assert(next_context != this);" just before
"this->swap_into(next_context);" in SwappedContext::suspend().