qemu: Check migratable host CPU features
    
    If calling query-cpu-model-expansion on the 'host'/'max' CPU model with
    'migratable' property set to false succeeds, we know QEMU is able to
    tell us which features would disable migration. Thus we can mark all
    enabled features as migratable.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>