Revert "Switch from yajl to Jansson"

This reverts commit 9cf38263d0.

Jansson cannot parse QEMU's quirky JSON.
Revert back to yajl.

https://bugzilla.redhat.com/show_bug.cgi?id=1614569

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemuagenttest.c

@@ -907,7 +907,7 @@ mymain(void)
 {
     int ret = 0;
 
-#if !WITH_JANSSON
+#if !WITH_YAJL
     fputs("libvirt not compiled with JSON support, skipping this test\n", stderr);
     return EXIT_AM_SKIP;
 #endif