boot-serial-test: use -no-shutdown

a qemu with an empty s390 guest will exit very quickly. This races
against the testsuite reading from the console pipe leading to
intermittent test suite failures. Using -no-shutdown will keep
the guest running.

Fixes: 864111f422ba (vl: exit qemu on guest panic if -no-shutdown is not set)
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1490361570-288658-1-git-send-email-borntraeger@de.ibm.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
