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 <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

agx/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,zippy2/libvirt,datto/libvirt,eskultety/libvirt,eskultety/libvirt,rlaager/libvirt,nertpinx/libvirt,elmarco/libvirt,olafhering/libvirt,eskultety/libvirt,taget/libvirt,crobinso/libvirt,datto/libvirt,libvirt/libvirt,rlaager/libvirt,taget/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,andreabolognani/libvirt,datto/libvirt,taget/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,shugaoye/libvirt,nertpinx/libvirt,datto/libvirt,jardasgit/libvirt,rlaager/libvirt,shugaoye/libvirt,andreabolognani/libvirt,shugaoye/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,taget/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,agx/libvirt,jfehlig/libvirt,taget/libvirt,olafhering/libvirt,fabianfreyer/libvirt,agx/libvirt,libvirt/libvirt,VenkatDatta/libvirt,elmarco/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,datto/libvirt,nertpinx/libvirt,rlaager/libvirt,olafhering/libvirt,elmarco/libvirt,jardasgit/libvirt,crobinso/libvirt,VenkatDatta/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,agx/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,agx/libvirt,andreabolognani/libvirt,olafhering/libvirt,elmarco/libvirt,shugaoye/libvirt,shugaoye/libvirt,eskultety/libvirt,rlaager/libvirt,elmarco/libvirt,jfehlig/libvirt