aio: Do aio_notify_accept only during blocking aio_poll

aio: Do aio_notify_accept only during blocking aio_poll

An aio_notify() pairs with an aio_notify_accept(). The former should
happen in the main thread or a vCPU thread, and the latter should be
done in the IOThread.

There is one rare case that the main thread or vCPU thread may "steal"
the aio_notify() event just raised by itself, in bdrv_set_aio_context()
[1]. The sequence is like this:

    main thread                     IO Thread
    ===============================================================
    bdrv_drained_begin()
      aio_disable_external(ctx)
                                    aio_poll(ctx, true)
                                      ctx->notify_me += 2
    ...
    bdrv_drained_end()
      ...
        aio_notify()
    ...
    bdrv_set_aio_context()
      aio_poll(ctx, false)
[1]     aio_notify_accept(ctx)
                                      ppoll() /* Hang! */

[1] is problematic. It will clear the ctx->notifier event so that
the blocked ppoll() will not return.

(For the curious, this bug was noticed when booting a number of VMs
simultaneously in RHV.  One or two of the VMs will hit this race
condition, making the VIRTIO device unresponsive to I/O commands. When
it hangs, Seabios is busy waiting for a read request to complete (read
MBR), right after initializing the virtio-blk-pci device, using 100%
guest CPU. See also https://bugzilla.redhat.com/show_bug.cgi?id=1562750
for the original bug analysis.)

aio_notify() only injects an event when ctx->notify_me is set,
correspondingly aio_notify_accept() is only useful when ctx->notify_me
_was_ set. Move the call to it into the "blocking" branch. This will
effectively skip [1] and fix the hang.

Furthermore, blocking aio_poll is only allowed on home thread
(in_aio_context_home_thread), because otherwise two blocking
aio_poll()'s can steal each other's ctx->notifier event and cause
hanging just like described above.

Cc: qemu-stable@nongnu.org
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20180809132259.18402-3-famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>