migration: Teach PSS about host page

Migration code has a lot to do with host pages.  Teaching PSS core about
the idea of host page helps a lot and makes the code clean.  Meanwhile,
this prepares for the future changes that can leverage the new PSS helpers
that this patch introduces to send host page in another thread.

Three more fields are introduced for this:

  (1) host_page_sending: this is set to true when QEMU is sending a host
      page, false otherwise.

  (2) host_page_{start|end}: these point to the start/end of host page
      we're sending, and it's only valid when host_page_sending==true.

For example, when we look up the next dirty page on the ramblock, with
host_page_sending==true, we'll not try to look for anything beyond the
current host page boundary.  This can be slightly efficient than current
code because currently we'll set pss->page to next dirty bit (which can be
over current host page boundary) and reset it to host page boundary if we
found it goes beyond that.

With above, we can easily make migration_bitmap_find_dirty() self contained
by updating pss->page properly.  rs* parameter is removed because it's not
even used in old code.

When sending a host page, we should use the pss helpers like this:

  - pss_host_page_prepare(pss): called before sending host page
  - pss_within_range(pss): whether we're still working on the cur host page?
  - pss_host_page_finish(pss): called after sending a host page

Then we can use ram_save_target_page() to save one small page.

Currently ram_save_host_page() is still the only user. If there'll be
another function to send host page (e.g. in return path thread) in the
future, it should follow the same style.

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

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