meta-armv8r64-extras/kas: append virtualization feature

Docker needs "virtualization" feature enabled, previous patch
appends this feature to DISTRO_FEATURES from a class in
IMAGE_CLASSES which does not take effect, this patch fix
this problem.

Issue-Id: SCM-4876
Signed-off-by: Jiacheng Tang <jiacheng.tang@arm.com>
Change-Id: I32700a5e4a34c11cc550c9c8fec8098e64eee148