tests: Use python selector in the parallel-vm.py main loop

tests: Use python selector in the parallel-vm.py main loop

This gets rid of the loop that was polling for things to do every 0.25
seconds and instead, reacts to any data from VMs as soon as it becomes
available. This avoids unnecessary operations when no new data is
available and avoids unnecessary waits when new data becomes available
more quickly.

Signed-off-by: Jouni Malinen <j@w1.fi>