util: pull Buffer code out of VNC module

The Buffer code in the VNC server is useful for the IO channel
code, so pull it out into a shared module, QIOBuffer.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
diff --git a/util/Makefile.objs b/util/Makefile.objs
index d8d7e7a..1363d1f 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -21,3 +21,4 @@
 util-obj-y += qemu-coroutine.o qemu-coroutine-lock.o qemu-coroutine-io.o
 util-obj-y += qemu-coroutine-sleep.o
 util-obj-y += coroutine-$(CONFIG_COROUTINE_BACKEND).o
+util-obj-y += buffer.o