qemu_agent: Produce more readable error messages

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>