Avoid accessing "closed" process in valgrind_memalign

During `vm.destroy()` the `vm.process` is closed, making it impossible
to access it's output (or pid, ...). Let's quit the machine manually
from the test, wait until it's dead and then verify userspace crash.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/valgrind_memalign.py

@@ -51,5 +51,6 @@ def run(test, params, env):
 
     error.context("Quit guest and check the process quit normally",
                   logging.info)
-    vm.destroy(gracefully=False)
+    vm.monitor.quit()
+    vm.wait_until_dead(5, 0.5, 0.5)
     vm.verify_userspace_crash()