migration: Drop postcopy_chunk_hostpages()

migration: Drop postcopy_chunk_hostpages()

This function calls three functions:

  - postcopy_discard_send_init(ms, block->idstr);
  - postcopy_chunk_hostpages_pass(ms, block);
  - postcopy_discard_send_finish(ms);

However only the 2nd function call is meaningful.  It's major role is to make
sure dirty bits are applied in host-page-size granule, so there will be no
partial dirty bits set for a whole host page if huge pages are used.

The 1st/3rd call are for latter when we want to send the disgard ranges.
They're mostly no-op here besides some tracepoints (which are misleading!).

Drop them, then we can directly drop postcopy_chunk_hostpages() as a whole
because we can call postcopy_chunk_hostpages_pass() directly.

There're still some nice comments above postcopy_chunk_hostpages() that explain
what it does.  Copy it over to the caller's site.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>