test: support disabling KVM by setting NO_KVM

test: support disabling KVM by setting NO_KVM

KVM fails on the Ubuntu ppc64el autopkgtest runners:

```
$ /usr/bin/qemu-system-ppc64el -enable-kvm -cpu host [...]
ioctl(KVM_CREATE_VM) failed: 22 Invalid argument
PPC KVM module is not loaded. Try modprobe kvm_hv.
qemu-system-ppc64el: failed to initialize kvm: Invalid argument
$ modprobe kvm_hv
modprobe: ERROR: could not insert 'kvm_hv': No such device
```

Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>