cpu: initialize cpu->exception_index on reset

This unbreaks linux-user (broken by e511b4d7, cpu-exec: reset exception_index
correctly, 2014-11-26).

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Tested-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 1418989994-17244-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>