docs: silence warnings about generated API docs

docs: silence warnings about generated API docs

I noticed these while testing 'make dist'.

Parsing ./../src/util/event.c
Function comment for virEventRegisterDefaultImpl lacks description of return value
Function comment for virEventRunDefaultImpl lacks description of return value
Parsing ./../src/util/virterror.c
Missing comment for function virSetErrorLogPriorityFunc

* src/util/event.c (virEventRegisterDefaultImpl)
(virEventRunDefaultImpl): Document return types.
* src/util/virterror.c (virSetErrorLogPriorityFunc): Provide docs.