virlog: Introduce virLogFindOutput

virlog: Introduce virLogFindOutput

Outputs are a bit trickier than filters, since the user(config)-specified
set of outputs can contain duplicates. That would lead to logging the same
message twice. For compatibility reasons, we cannot just error out and forbid
the daemon to start if we find duplicate outputs which do not make sense.
Instead, we could silently take into account only the last occurrence of the
duplicate output and remove all the previous ones, so that the logger will not
try to use them when it is looping over all of its registered outputs.

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