Fix qemu_target.py to kill QEMU process on failure.

Previously if QEMU ran but neither exited prematurely, nor did the
guest ever become connectible via SSH, then the QEMU sub-process would
be left running after the runner script exited.

Bug: 877194
Change-Id: Id50fa44e11987f69afc469d73adc445b5066b7a5
Reviewed-on: https://chromium-review.googlesource.com/1187543
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585709}

build/fuchsia/qemu_target.py

@@ -51,8 +51,9 @@ class QemuTarget(target.Target):
   # Used by the context manager to ensure that QEMU is killed when the Python
   # process exits.
   def __exit__(self, exc_type, exc_val, exc_tb):
-    if self.IsStarted():
-      self.Shutdown()
+    if self._IsQemuStillRunning():
+      logging.info('Shutting down QEMU.')
+      self._qemu_process.kill()
 
   def Start(self):
     qemu_path = os.path.join(QEMU_ROOT, 'bin',
@@ -154,10 +155,6 @@ class QemuTarget(target.Target):
                                           stdout=stdout, stderr=stderr)
     self._WaitUntilReady();
 
-  def Shutdown(self):
-    logging.info('Shutting down QEMU.')
-    self._qemu_process.kill()
-
   def _IsQemuStillRunning(self):
     return os.waitpid(self._qemu_process.pid, os.WNOHANG)[0] == 0