fix redirection of STDOUT/ERR of ssh port forwarding process (#226)

The existing invocation fails to spawn correctly, and even if that
were the case, redirecting STDOUT/STDERR to /dev/null risks losing
valuable debug, as seen in #225.

Closes #226.

https://github.com/pradels/vagrant-libvirt/issues/226
Merge pull request #264 from aspiers/ssh-forward-logging

fix redirection of STDOUT/ERR of ssh port forwarding process (#226)