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/qemublocktest.c

@@ -309,7 +309,6 @@ testQemuDiskXMLToPropsValidateFile(const void *opaque)
             goto cleanup;
 
         virBufferAdd(&buf, jsonstr, -1);
-        virBufferAddLit(&buf, "\n");
         VIR_FREE(jsonstr);
     }