throttle: Add a new throttling API implementing continuous leaky bucket.

Implement the continuous leaky bucket algorithm devised on IRC as a separate
module.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/util/Makefile.objs b/util/Makefile.objs
index dc72ab0..2bb13a2 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -11,3 +11,4 @@
 util-obj-y += qemu-option.o qemu-progress.o
 util-obj-y += hexdump.o
 util-obj-y += crc32c.o
+util-obj-y += throttle.o