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 branches 'preexisting-disks', 'fix/disk-path' and 'extra-storage-cache-mode' into disks

Conflicts:
	README.md
	lib/vagrant-libvirt/config.rb
Merge branches 'allow-newer-ruby-libvirt', 'remove-default-provider', 'ssh-forward-logging' and 'disks' into working