build: fix VIR_DEBUG on mingw

build: fix VIR_DEBUG on mingw

We don't use the gnulib vsnprintf replacement, which means that
on mingw, vsnprintf doesn't support %zn or %lln.

And as it turns out, VIR_GET_VAR_STR was a rather inefficient
reimplementation of virVasprintf logic.

* src/util/logging.c (VIR_GET_VAR_STR): Drop.
(virLogMessage): Inline a simpler version here.
* src/util/virterror.c (VIR_GET_VAR_STR, virRaiseErrorFull):
Likewise.
Reported by Matthias Bolte.