def load_host_keys(self, filename):
self._host_keys_filename = filename<EOL>self._host_keys.load(filename)<EOL>
Load host keys from a local host-key file.  Host keys read with this
method will be checked I{after} keys loaded via L{load_system_host_keys},
but will be saved back by L{save_host_keys} (so they can be modified).
The missing host key policy L{AutoAddPolicy} adds keys to this set and
saves them, when connecting to a previously-unknown server.

This method can be called multiple times.  Each new set of host keys
will be merged with the existing set (new replacing old if there are
conflicts).  When automatically saving, the last hostname is used.

@param filename: the filename to read
@type filename: str

@raise IOError: if the filename could not be read