I have a Lenovo ThinkPad T430 running Red Hat Enterprise Linux 6.7 with kernel 2.6.32-573.8.1.el6.x86_64. This is a work computer and my corporate policy states that I must keep my operating system up to date, so I recently installed the aforementioned kernel version via yum upgrade (the upgrade also pulled over 100 updated packages), restarted, and now I was unable to start my VMs. Attempting to start one from virt-manager or with virsh --connect qemu:///system list throws an error message that says error: internal error Cannot parse sensitivity level in s0.I eventually managed to fix this error as follows in the answer:
HOWTO: Fix Cannot parse sensitivity level in s0 error when attempting to start a KVM VM after a major OS upgrade
rhel;kvm;qemu;selinux
Googling around I found this RHEL error report. Now, I know pretty little about SELinux but I do remember seeing a SELinux relabel taking place after upgrading my OS distribution, my SELinux issues casually happened after a kernel upgrade and the error report suggested trying a forced autorelabel by creating an empty file on the system root directory called .autorelabel.So I created that file, restarted my computer, waited for the autorelabel to complete, and tried running a VM again. My VMs now start normally and the problem is fixed.