targetcontrol: Improve logs by outputting qemu boot log in case of failure

We've had a few cases on the autobuilder where we've lost logs of
the boot and been unable to debug it further. Show this information
onto the console to be more useful.

(From OE-Core rev: 5302b7b8fa1349ae99040fe8fdcc4e42c722cc10)

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

meta/lib/oeqa/targetcontrol.py

@@ -175,6 +175,9 @@ class QemuTarget(BaseTarget):
             self.connection = SSHControl(ip=self.ip, logfile=self.sshlog)
         else:
             self.stop()
+            if os.path.exists(self.qemulog):
+                with open(self.qemulog, 'r') as f:
+                    bb.error("Qemu log output from %s:\n%s" % (self.qemulog, f.read()))
             raise bb.build.FuncFailed("%s - FAILED to start qemu - check the task log and the boot log" % self.pn)
 
     def stop(self):