qemu: only allow enabling deprecated features that are supported

qemu: only allow enabling deprecated features that are supported

When updating the guest CPU model and the deprecated_features attribute
is set to on, only enable the features the model can actually enable.

While host-model would normally just enable these features without
intervention (and without the presence of the deprecated_features
attribute), custom models would see no changes to their feature set
without these changes.

This is useful for e.g. testing CPU models.

Fixes: f279ea36 (qemu: process: refactor deprecated features code)
Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>