conf: add control over COW for storage pool directories

conf: add control over COW for storage pool directories

The storage pool code now attempts to disable COW by default on btrfs,
but management applications may wish to override this behaviour. Thus we
introduce a concept of storage pool features:

  <features>
    <cow state='yes|no'/>
  </features>

If the <cow> feature policy is set, it will be enforced. It will always
return an hard error if COW cannot be explicitly set or unset.

Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>