ipmi_bmc_sim: Add break to correct watchdog NMI check

It was falling through when it should have been a break.  Found by
Coverity.  The logic could be simplified a bit with a fallthrough,
probably the original thought, but that would be less clear, I think.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Shannon Zhao <zhaoshenglong@huawei.com>
Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Message-Id: <1452519152-6500-3-git-send-email-minyard@acm.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>