virlog: Convert virLogOutputs to a list of pointers to outputs

virlog: Convert virLogOutputs to a list of pointers to outputs

Right now, we define outputs one after another. However, the correct flow
should be to define a set of outputs as a whole unit. Therefore each output
should be first created, placed into an array/list and the list will be
defined. Output creation should be a separate operation, so an output will be
returned by a reference. From that perspective, it makes perfect sense to
only store pointers to actual outputs.

Signed-off-by: Erik Skultety <eskultet@redhat.com>