[libvirt] Fix cygwin build

In virLogEmergencyDumpAll I was using SIGBUS and SIGUSR2, and apparently they arent; defined on cygwin signal.h header. I decided to make the code conditional for all of them and not just those two for portability sake. I resisted doing this on the daemon side because as far as I know we don't compile it on WIN32. So I just pushed the following under the build breaking rule, Daniel -- Daniel Veillard | libxml Gnome XML XSLT toolkit http://xmlsoft.org/ daniel(a)veillard.com | Rpmfind RPM search engine http://rpmfind.net/ http://veillard.com/ | virtualization library http://libvirt.org/