migration: Do chunk page in postcopy_each_ram_send_discard()

migration: Do chunk page in postcopy_each_ram_send_discard()

Right now we loop ramblocks for twice, the 1st time chunk the dirty bits with
huge page information; the 2nd time we send the discard ranges.  That's not
necessary - we can do them in a single loop.

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>