The overhead of the mutex in non-multifd mode is negligible,
because in that case its just the single thread taking the mutex.

This will be used in the next commits to add colo support to multifd.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <22d83cb428f37929563155531bfb69fd8953cc61.1683572883.git.lukasstraub2@web.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>