qemu_agent: Produce more readable error messages
    
    Not every error message from qemu-ga has to have the 'class' field
    filled out. For instance, I've seen this error message lately:
    
      qemuAgentCheckError:1047 : unable to execute QEMU agent command \
      {"execute":"guest-set-time"}: \
      {"error":{"desc":"Invalid parameter type, expected: integer"}}
    
    However, this got translated into rather generic error message:
    
      internal error: unable to execute QEMU agent command
      'guest-set-time': unknown QEMU command error
    
    So we've dropped better error message in favor of a generic one.
    This is due to our code which expects 'class' which is not
    present here.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>