xci: scripts: start-new-vm.sh: Disable nested virtualization in clean VM

xci: scripts: start-new-vm.sh: Disable nested virtualization in clean VM

When using the clean VM for the tests, we will end up with 3 levels of
virtualization when functest is executed (1st level = clean vm, 2nd
level = compute node, 3rd level = functest VMs). This makes the functest
VM terribly slow leading to all sorts of random failures. It appears
that KVM can't handle this case, so we need to switch to the QEMU
interpreter instead which is slower but functional. This only affects
the Jenkins jobs, so deployments on baremetal will still use 2 levels of
nested KVM virtualization which should work fine.

Change-Id: If274129fbf347526982ac4cf577d216173eb4d1b
Signed-off-by: Markos Chandras <mchandras@suse.de>