tests/boot-serial: Do not delete the output file in case of errors

tests/boot-serial: Do not delete the output file in case of errors

Peter reported that the boot-serial tester sometimes runs into timeouts
with SPARC guests. It's currently completely unclear whether this is due
to too much load on the host machine (so that the guest really just ran
too slow), or whether there is something wrong with the guest's firmware
boot. For further debugging, we need the serial output of the guest in
case of errors, so instead of unlinking the file immediately, this is
now only done in case of success. In case of error, print the name of the
file with the serial output via g_error() (which then also calls abort()
internally to mark the test as failed).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1526977831-31129-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>