[CentOS] SELinux and KVM

>  Of course, virt-manager now does not work.  It opens but it does
> not provide any means of adding a new virtual host.

What are the symptoms?
Does virt-manager ask for your root password when starting?

Did you try with SELinux in permissive mode?

I recommend that you install setroubleshoot, it helps to debug SELinux
issues, and will be notified when in permissive mode as well:
sudo yum install setroubleshoot