Vulnerability CVE–2016–5195 deals with privilege escalation in Linux systems.
In a virtual environment with hypervisor KVM and QEMU, can a virtual machine get access to it's host? If not, what is the protective layer here?
You are looking at two different vulnerabilities.
Dirty Cow is a privilege escalation in the operating system. So, if your VM is vulnerable, that means it's possible to get root privileges in the VM's operating system, not in the host system.
In order to be possible to access the host, you need a vulnerability in KVM or QEMU that allows you to escape from the VM sandbox.