Merge remote-tracking branch 'quintela/thread.next' into staging

# By Juan Quintela (7) and Paolo Bonzini (6)
# Via Juan Quintela
* quintela/thread.next:
  migration: remove argument to qemu_savevm_state_cancel
  migration: Only go to the iterate stage if there is anything to send
  migration: unfold rest of migrate_fd_put_ready() into thread
  migration: move exit condition to migration thread
  migration: Add buffered_flush error handling
  migration: move beginning stage to the migration thread
  qemu-file: Only set last_error if it is not already set
  migration: fix off-by-one in buffered_rate_limit
  migration: remove double call to migrate_fd_close
  migration: make function static
  use XFER_LIMIT_RATIO consistently
  Protect migration_bitmap_sync() with the ramlist lock
  Unlock ramlist lock also in error case