tests/qtest/boot-sector: Check that the guest did not panic

The s390-ccw bios code panics if it can not boot successfully. In
this case, it does not make sense that we wait the full 600 seconds
for the boot sector test to finish and can signal the failure
immediately, thus let's check the status of the guest with the
"query-status" QMP command here, too.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210212113141.854871-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>