Actually many of the VM exploits are related to qemu device emulation or paravirtualization drivers, which are closed by the use of Xen stubdom. Only very few were privilege escalation via another vector, both in kvm and in Xen. I have no idea about other hypervisors.
And in turn most QEMU vulnerabilities are closed by SELinux of your distribution enables it. Libvirt (and thus the virt-manager GUI) automatically confine each QEMU process so that it can only access the resources for that particular VM.
