qemu: Fix qemuMonitorCreateObjectProps

qemu: Fix qemuMonitorCreateObjectProps

Fix the return value status comparison checking for call to
virJSONValueObjectCreateVArgs introduced by commit id f0a23c0c3.

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: John Ferlan <jferlan@redhat.com>