QEMUFileBuffered: indicate that we're ready when the underlying file is ready

QEMUFileBuffered: indicate that we're ready when the underlying file is ready

QEMUFileBuffered stops writing when the underlying QEMUFile is not ready,
and tells its producer so.  However, when the underlying QEMUFile becomes
ready, it neglects to pass that information along, resulting in stoppage
of all data until the next tick (a tenths of a second).

Usually this doesn't matter, because most QEMUFiles used with QEMUFileBuffered
are almost always ready, but in the case of exec: migration this is not true,
due to the small pipe buffers used to connect to the target process.  The
result is very slow migration.

Fix by detecting the readiness notification and propagating it.  The detection
is a little ugly since QEMUFile overloads put_buffer() to send it, but that's
the suject for a different patch.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 5e77aaa0d7d2f4ceaa4fcaf50f3a26d5150f34a6)