migration/xbzrle: update cache and current_data in one place

migration/xbzrle: update cache and current_data in one place

When we are not in the last_stage, we need to update the cache if page
is not the same.

Currently this procedure is scattered in two places and mixed with
encoding status check.

This patch extract this general step out to make the code a little bit
easy to read.

Signed-off-by: Wei Yang <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20190610004159[email protected]>
Signed-off-by: Juan Quintela <[email protected]>