postcopy: Send whole huge pages

The RAM save code uses ram_save_host_page to send whole
host pages at a time;  change this to use the host page size associated
with the RAM Block which may be a huge page.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20170224182844.32452-12-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>