[QA] guest users have full read/write permission on all external storages by default

[jnweiger]

Seen with guests-0.9.1 on server 10.7.0

When an external drive is configured without group/user limitation, guests gain full r/w access
In this example guests have access to the WND drive, but not SFTP, where we manually limited to users group.

Enhancement suggestion: per default exclude guests group, unless the guests group is explictily added.