Drop unused return value of virLogOutputFunc

Drop unused return value of virLogOutputFunc

Nothing uses the return value, and creating it requries otherwise
unnecessary strlen () calls.

This cleanup is conceptually independent from the rest of the series
(although the later patches won't apply without it).  This just seems
a good opportunity to clean this up, instead of entrenching the unnecessary
return value in the virLogOutputFunc instance that will be added in this
series.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>