kernel-test: don't try to use KVM if not available
Disable KVM if /dev/kvm is missing, as it is apparently the case on our jenkins nodes. This makes the tests run a bit slower, but not much. Add kernel_test_wait_for_vm to sleep until the VM is booted up (~5s instead of ~1s without KVM), so the first test in the testsuite does not fail when using KVM.
The variable is useful to test the non-KVM code path even if KVM is available on the host.
