[Libguestfs] simplify debugging of guestfsd

Sometimes guestfsd fails to gather info, and the virt-<tool> -v output is usually not useful to figure out whats going on within the temporary guest. I see the /init script has support to run guestfsd with a debug tool, which is currently valgrind. Granted, valgrind support is a compile time thing. What would be a good way to optionally run guestfsd with strace or gdb? Should each tool get a new option, something like 'virt-<tool> --debug=strace', or should it be like 'XYZ_STRACE=1 virt-<tool>'? Olaf