migration: Synchronize multifd threads with main thread

migration: Synchronize multifd threads with main thread

We synchronize all threads each RAM_SAVE_FLAG_EOS.  Bitmap
synchronizations don't happen inside a  ram section, so we are safe
about two channels trying to overwrite the same memory.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
--
seq needs to be atomic now, will also be accessed from main thread.
Fix the if (true || ...) leftover
We are back to non-atomics