migration/postcopy: not necessary to do discard when canonicalizing bitmap

migration/postcopy: not necessary to do discard when canonicalizing bitmap

All pages, either partially sent or partially dirty, will be discarded in
postcopy_send_discard_bm_ram(), since we update the unsentmap to be
unsentmap = unsentmap | dirty in ram_postcopy_send_discard_bitmap().

This is not necessary to do discard when canonicalizing bitmap. And by
doing so, we separate the page discard into two individual steps:

  * canonicalize bitmap
  * discard page

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190819061843.28642-2-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>