Just in case this is useful, I'd like to submit this change below that was not present
at least in 1.0.4
http://git.onelab.eu/?p=libvirt.git;a=blob;f=virgetlasterror.patch;h=9109...
I'd taken this hint here in the first place
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=700077
-- Thierry