Code improvements

[jjnicola]

What:

1. Set host end_time first and then set the host finished.
2. Log a message if the scanner did not launched all plugins against a host.
3. Do not call pluginlaunch_stop() twice.

Why:

1. this is the order which ospd-openvas expects the status.
2. To know if not all plugins were launched against the host.
3. Therefore.

How:

Checklist:

• Tests
• CHANGELOG Entry