QMP: Small cleanup in handle_qmp_command()

QMP has its own dispatch tables, we can now drop the following
checks:

    o 'info' command: this command doesn't exist in QMP's
       dispatch table, the right thing will happen when it's
       issued by a client (ie. command not found error)

    o monitor_handler_ported(): all QMP handlers are 'ported', no
      need to check for that

    o monitor_cmd_user_only(): no HMP handler will exist in QMP's
      dispatch tables, that's why we have split them after all :-)

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
