runqemu: Add a hook to allow it to renice

We have an issue where qemu is being starved of resources on our autobuilders.
We can't raise its priority without special capacilties, therefore add a hook
which if present can allow this to happen using an executable
"~/runqemu-renice".

(From OE-Core rev: 829443b3fafa000e330c06ebb144641bd2d20ad1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 141a3c9ce93bc3d526303021ecf0460c6e9fea8a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -1507,6 +1507,11 @@ def main():
     try:
         config = BaseConfig()
 
+        renice = os.path.expanduser("~/bin/runqemu-renice")
+        if os.path.exists(renice):
+            logger.info('Using %s to renice' % renice)
+            subprocess.check_call([renice, str(os.getpid())])
+
         def sigterm_handler(signum, frame):
             logger.info("SIGTERM received")
             os.kill(config.qemupid, signal.SIGTERM)