oeqa/qemurunner: Fix AttributeError: QemuRunner instance has no attribute 'server_socket'

If start() returns False due to create_socker() failing, stop() may still get
called and currently this gives a track back since server_socket doesn't exist.

Avoid this.

(From OE-Core rev: 26fb69ec663afd05134822dc5e48fc8928ea5bf1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/utils/qemurunner.py

@@ -190,7 +190,7 @@ class QemuRunner:
                 logger.info("Sending SIGKILL to runqemu")
                 os.killpg(self.runqemu.pid, signal.SIGKILL)
             self.runqemu = None
-        if self.server_socket:
+        if hasattr(self, 'server_socket') and self.server_socket:
             self.server_socket.close()
             self.server_socket = None
         self.qemupid = None