rpc: libssh: allow a NULL known_hosts file
    
    Make sure that virNetLibsshSessionSetHostKeyVerification accepts a NULL
    value for the path to the known_hosts file:
    - call ssh_options_set(SSH_OPTIONS_KNOWNHOSTS) anyway, using /dev/null,
      otherwise libssh will use its default path
    - do not call ssh_write_knownhost when no known hosts file was set
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1406457