acpi-build: fix build on glib < 2.22 g_string_vprintf was only introduced in 2.24 so switch to vsnprintf instead. A bit uglier but name size is fixed at 4 bytes here so it's easy. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Reported-by: Richard Henderson <rth@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>