qemu: Fix qemuMonitorCreateObjectProps

Fix the return value status comparison checking for call to
virJSONValueObjectCreateVArgs introduced by commit id f0a23c0c.

If a NULL arglist is passed, then a 0 is returned which is a
valid status and we only should fail when the return is < 0.

This resolves an issue seen for "virsh iothreadadd $dom $iothread"
where a "error: An error occurred, but the cause is unknown" error
was generated when trying to hotplug an IOThread to a domain since
qemuDomainHotplugAddIOThread passes a NULL arglist.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_monitor.c

@@ -3051,7 +3051,7 @@ qemuMonitorCreateObjectProps(virJSONValuePtr *propsret,
 
     va_start(args, alias);
 
-    if (!(virJSONValueObjectCreateVArgs(&props, args)))
+    if (virJSONValueObjectCreateVArgs(&props, args) < 0)
         goto cleanup;
 
     if (!(*propsret = qemuMonitorCreateObjectPropsWrap(type, alias, &props)))