We don't use gnulib's sanitizations for vfprintf, but vshDebug
was used with %zu, which means that it would fail on mingw.
Thank goodness the compiler indirectly caught this for us :)

virsh.c: In function 'vshDebug':
virsh.c:12105:5: warning: function might be possible candidate for
'ms_printf' format attribute [-Wmissing-format-attribute]

since mingw <stdio.h> hasn't yet added gcc attributes to vfprintf.

* tools/virsh.c (vshDebug): Avoid vfprintf.
(vshPrintExtra): Use lighter-weight fputs.
Reported by Matthias Bolte.