net: do not use atexit for cleanup

This will be necessary in the next patch, which stops using atexit for
character devices; without it, vhost-user and the redirector filter
will cause a use-after-free.  Relying on the ordering of atexit calls
is also brittle, even now that both the network and chardev
subsystems are using atexit.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
