slirp: use exit notifier for slirp_smb_cleanup

slirp: use exit notifier for slirp_smb_cleanup

We would like to move back net_cleanup() at the end of main function,
like it used to be until f30dbae63a46f23116715dff8d130c, but minimum
cleanup is needed regardless at exit() time for slirp's SMB
functionality.  Use an exit notifier to call slirp_smb_cleanup.
If net_cleanup() is called first, then remove the exit notifier as it
will become a dangling pointer otherwise.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>