Fix ipa migrate-ds when it finds a search reference
    
    When ipa migrate-ds finds user entries and a search reference, it complains
    that the LDAP search did not return any result and does not migrate the
    entries or the groups.
    
    The issue comes from LDAPClient._convert_result which returns an empty result
    list when the input is a search reference. In turn LDAPClient.find_entries
    assumes that the empty result list corresponds to a Search Result Done and
    returns without any entry.
    
    The fix examines first the objtype returned by self.conn.result3. If it is
    a search result done, then the loop can be exited. Otherwise (referral or
    entry), _convert_result is called and the result (if not empty) is appended
    to the list of returned entries.
    
    https://fedorahosted.org/freeipa/ticket/6358
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>