qemu.py: refactor launch()

This is just a refactor to separate the exception handler from the
actual launch procedure, improving the readability and making future
maintenances in this piece of code easier.

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20180122205033.24893-3-apahim@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

scripts/qemu.py

@@ -205,20 +205,14 @@ def _post_shutdown(self):
             self._temp_dir = None
 
     def launch(self):
-        '''Launch the VM and establish a QMP connection'''
+        """
+        Launch the VM and make sure we cleanup and expose the
+        command line/output in case of exception
+        """
         self._iolog = None
         self._qemu_full_args = None
-        devnull = open(os.path.devnull, 'rb')
         try:
-            self._pre_launch()
-            self._qemu_full_args = (self._wrapper + [self._binary] +
-                                    self._base_args() + self._args)
-            self._popen = subprocess.Popen(self._qemu_full_args,
-                                           stdin=devnull,
-                                           stdout=self._qemu_log_file,
-                                           stderr=subprocess.STDOUT,
-                                           shell=False)
-            self._post_launch()
+            self._launch()
         except:
             if self.is_running():
                 self._popen.kill()
@@ -233,6 +227,19 @@ def launch(self):
                 LOG.debug('Output: %r', self._iolog)
             raise
 
+    def _launch(self):
+        '''Launch the VM and establish a QMP connection'''
+        devnull = open(os.path.devnull, 'rb')
+        self._pre_launch()
+        self._qemu_full_args = (self._wrapper + [self._binary] +
+                                self._base_args() + self._args)
+        self._popen = subprocess.Popen(self._qemu_full_args,
+                                       stdin=devnull,
+                                       stdout=self._qemu_log_file,
+                                       stderr=subprocess.STDOUT,
+                                       shell=False)
+        self._post_launch()
+
     def wait(self):
         '''Wait for the VM to power off'''
         self._popen.wait()