remote: do not check for an existing config dir
    
    When composing the path to the default known_hosts file (for the libssh
    and libssh2 drivers), do not check whether the configuration directory
    (determined by virGetUserConfigDirectory()) exists: both the drivers can
    handle non-existing files, and are able to create them (and their
    directories) in that case.
    
    This adds a small behaviour change: before, the key for an unknown host,
    and manually accepted, was saved only if the configuration directory
    existed -- a bit incoherent behaviour though.