vnc: refuse to set a password with VNC_AUTH_NONE

Current code silently changes the authentication settings
in case you try to set a password without password authentication
turned on.  This is bad.  Return an error instead.

If we want allow changing auth settings at runtime this should
be done explicitly using a separate monitor command, not as
side effect of set_passwd.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
