util: add base64 decoding function

The standard glib provided g_base64_decode doesn't provide any
kind of sensible error checking on its input. Add a QEMU custom
wrapper qbase64_decode which can be used with untrustworthy
input that can contain invalid base64 characters, embedded
NUL characters, or not be NUL terminated at all.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
diff --git a/util/Makefile.objs b/util/Makefile.objs
index 89dd80e..8620a80 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -30,3 +30,4 @@
 util-obj-y += coroutine-$(CONFIG_COROUTINE_BACKEND).o
 util-obj-y += buffer.o
 util-obj-y += timed-average.o
+util-obj-y += base64.o