migration: Create multifd migration threads

migration: Create multifd migration threads

Creation of the threads, nothing inside yet.

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

Use pointers instead of long array names
Move to use semaphores instead of conditions as paolo suggestion

Put all the state inside one struct.
Use a counter for the number of threads created.  Needed during cancellation.

Add error return to thread creation

Add id field

Rename functions to multifd_save/load_setup/cleanup
Change recv parameters to a pointer to struct
Change back to a struct
Use Error * for _cleanup