migration: move rate limiting to QEMUFile

Rate limiting is now simply a byte counter; client call
qemu_file_rate_limit() manually to determine if they have to exit.
So it is possible and simple to move the functionality to QEMUFile.

This makes the remaining functionality of s->file redundant;
in the next patch we can remove it and write directly to s->migration_file.

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>