migration: Fix regression with compression threads

Compression threads got broken on commit

  commit 247956946651ae0280f7b1ea88bb6237dd01c231
  Author: Juan Quintela <quintela@redhat.com>
  Date:   Tue Mar 21 11:45:01 2017 +0100

      ram: reorganize last_sent_block

On do_compress_ram_page() we use a different QEMUFile than the
migration one.  We need to pass it there.  The failure can be seen as:

(qemu) qemu-system-x86_64: Unknown combination of migration flags: 0
qemu-system-x86_64: error while loading state section id 3(ram)
qemu-system-x86_64: load of migration failed: Invalid argument

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Peter Xu <peterx@redhat.com>
