nodeinfo: Phase out cpu_set_t usage
    
    Swap out all instances of cpu_set_t and replace them with virBitmap,
    which some of the code was already using anyway.
    
    The changes are pretty mechanical, with one notable exception: an
    assumption has been added on the max value we can run into while
    reading either socket_it or core_id.
    
    While this specific assumption was not in place before, we were
    using cpu_set_t improperly by not making sure not to set any bit
    past CPU_SETSIZE or explicitly allocating bigger bitmaps; in fact
    the default size of a cpu_set_t, 1024, is way too low to run our
    testsuite, which includes core_id values in the 2000s.