build: work around older systemtap header

build: work around older systemtap header

Systemtap 1.2 <sys/sdt.h> tried to expand STAP_PROBE3 into an
initialization:
  volatile __typeof__(arg) foo = arg;
but that fails if arg was declared as 'char arg[100]'.
Rather than make all callers to PROBE deal with the stupidity
of <sys/sdt.h>, we instead make PROBE cast away the problem.
Some of this preprocessor abuse copies ideas in src/libvirt.c.

* daemon/libvirtd.h (PROBE): Add casts to all arguments, using...
(VIR_ADD_CASTS, VIR_ADD_CAST, VIR_ADD_CAST2, VIR_ADD_CAST3)
(VIR_ADD_CAST_EXPAND, VIR_ADD_CAST_PASTE, VIR_COUNT_ARGS)
(VIR_ARG5, PROBE_EXPAND): New macros.
Reported by Wen Congyang.