guest_vsnprintf
int guest_vsnprintf(char *buf, int n, const char *fmt, va_list args);
len = guest_vsnprintf(buf, n, fmt, va);
guest_vsnprintf(uc->buffer, UCALL_BUFFER_LEN, fmt, va);
guest_vsnprintf(uc->buffer, UCALL_BUFFER_LEN, fmt, va);