Enforce return check on virAsprintf() calls

Enforce return check on virAsprintf() calls

Way back when I started making changes for Coverity messages my first set
were to a bunch of CHECKED_RETURN errors.  In particular virAsprintf() had
a few callers that Coverity noted didn't check their return (although some
did check if the buffer being printed to was NULL or not).

It was suggested at the time as a further patch an ATTRIBUTE_RETURN_CHECK
should be added to virAsprintf(), see:

https://www.redhat.com/archives/libvir-list/2013-January/msg00120.html

This patch does that and fixes a few more instances not found by Coverity
that failed the check.