return error from guest-agent

return error from guest-agent

in case of e.g. a non-existant guest-agent command, it would return
{ error: {someerrorobject} }
but we did only include the 'return' property

in case we do not get any and the error property is set,
return that

i looked at all the paths were we use the QMPClient, and either
we have our own callback for the result,
or we do not rely on the result being empty upon an error, so this
should not break anything

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>