Don't double-encode ssh key for XML output
Make ssh1 and ssh2 fetch_host_key return the same structure

This was indirectly causing the crash reported here:
http://seclists.org/nmap-dev/2014/q4/332

ssh1.fetch_host_key now returns a table with the full_key value
containing a key as it would be output by ssh-keygen. The previous value
named full_key is now named "key".