tests/acceptance: Don't test reboot on cubieboard

tests/acceptance: Don't test reboot on cubieboard

cubieboard does not have a functioning reboot, it halts and QEMU does
not exit.

vm.shutdown() is modified in a forthcoming patch that makes it less tolerant
of race conditions on shutdown; tests should consciously decide to WAIT
or to SHUTDOWN qemu.

So long as this test is attempting to reboot, the correct choice would
be to WAIT for the VM to exit. However, since that's broken, we should
SHUTDOWN instead.

SHUTDOWN is indeed what already happens when the test performs teardown,
however, if anyone fixes cubieboard reboot in the future, this test will
develop a new race condition that might be hard to debug.

Therefore: remove the reboot test and make it obvious that the VM is
still running when the test concludes, where the test teardown will do
the right thing.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20200710050649.32434-10-jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>