[Libguestfs] simplify debugging of guestfsd

[Olaf Hering]

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