qemu: monitor: Fix device matching in qemuMonitorJSONBlockIoThrottleInfo
We should compare the alias/qdev id only when it was provided by the
caller and when it was found in the reply. Otherwise we could
dereference a NULL pointer. STRNEQ_NULLABLE is not appropriate since
it would return 'true' if the string was not present in the JSON output.
Found by Coverity.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: John Ferlan <jferlan@redhat.com>