qemu: monitor: Open-code retrieval of wr_highest_offset
    
    Instead of using qemuMonitorJSONDevGetBlockExtent (which I plan to
    remove later) extract the data in place.
    
    Additionally add a flag that will be set when the wr_highest_offset was
    extracted correctly so that callers can act according to that.
    
    The test case addition should help make sure that everything works.