Compression code (still not on tree) want to call this funtion from
outside the migration thread, so we can't write to last_sent_block.

Instead of reverting full patch:

[PULL 07/11] save_block_hdr: we can recalculate

Just revert the parts that touch last_sent_block.

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