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.