spread: deal with crashing qemu in waitPortUp()

[mvo5]

Someones qemu crashes or dies while starting up. If that happens spread does not detected it and keeps trying to connect to the port until it times out. This leaves the user in the dark why the connection did not happen.

This commit improves this in two ways:

1. Show the stdout/stderr output of qemu on the terminal to ensure that users are aware of any issues there.
2. Add a check in waitPortUp() to detect if the command that is responsible for the setting up of the port exited in an unexpected way.