Some diagnostics had a hard-coded "libvirtd: " prefix, some used
"error: " and some used "argv[0]: ".  Always use "argv[0]: ".
* daemon/libvirtd.c (argv0): New global.
(main): Set it.
(version, usage): Remove argv0 parameter.  Use global; update callers.
(daemonForkIntoBackground): Use argv0:, not error:.
(qemudWritePidFile): Start each diagnostic with argv0:.
Suggested by Eric Blake.