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>