This patch enables vCPU notification to wake it up
when new async event comes in replay mode.

The motivation of this patch is the following.
Consider recorded block async event. It is saved into the log
with one of the checkpoints. This checkpoint may be passed in
vCPU loop. In replay mode when this async event is read from
the log, and block thread task is not finished yet, vCPU thread
goes to sleep. That is why this patch adds waking up the vCPU
to process this finished event.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Message-Id: <161726519158.1476949.7614181684462079836.stgit@pasha-ThinkPad-X280>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>