oeqa: Test failure/cleanup improvements

Currently, if qemu segfaults, the tests merrily continue trying to execute
which takes time for them to timeout and is a bit silly. Worse, no logs about
the segfault are shown to the user, its silent!

This patch tries to unravel the tangled web of issues and ensures that we:

* install a SIGCHLD handler which tells the user qemu exited
* check if qemu is running, if it isn't fail the test outright
* don't leave processes behind in sshcontrol which would hold
  bitbake.lock and block shutdown

(From OE-Core rev: 77e9363feba53b72429154be5713c46b007ae0a4)

(From OE-Core rev: c77b57e2fa8a4cf90543038fe1939f6ca9b12bd1)

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

meta/lib/oeqa/utils/qemurunner.py

@@ -280,6 +280,7 @@ class QemuRunner:
             self.server_socket = None
         self.qemupid = None
         self.ip = None
+        signal.signal(signal.SIGCHLD, self.origchldhandler)
 
     def stop_thread(self):
         if self.thread and self.thread.is_alive():