qtest: use synchronous I/O for char device

Peter reported that rtc-test would periodically hang.  It turns out
this was due to an EAGAIN occurring on qemu_chr_fe_write.

Instead of heavily refactoring qtest, just use a synchronous version
of the write operation for qemu_chr_fe_write to address this problem.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qtest.c

@@ -191,7 +191,7 @@ static void GCC_FMT_ATTR(2, 3) qtest_send(CharDriverState *chr,
     len = vsnprintf(buffer, sizeof(buffer), fmt, ap);
     va_end(ap);
 
-    qemu_chr_fe_write(chr, (uint8_t *)buffer, len);
+    qemu_chr_fe_write_all(chr, (uint8_t *)buffer, len);
     if (qtest_log_fp && qtest_opened) {
         fprintf(qtest_log_fp, "%s", buffer);
     }