migration: Abstract the number of bytes sent

Right now we use the "position" inside the QEMUFile, but things like
RDMA already do weird things to be able to maintain that counter
right, and multifd will have some similar problems.

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