libqos virtio: Increase ISR timeout

Increase the clock step to avoid Travis failure in some builds due to
overagressive timeout.

Signed-off-by: Marc Marí <marc.mari.barcelo@gmail.com>
Message-id: 1410428416-5046-1-git-send-email-marc.mari.barcelo@gmail.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>