Fix the node powered field

When initially implementing the node query, I thought the 'powered'
field is a representation of the run-time powered status, which would
make its query complex.

In reality, it's a simple config query, which we can support
easily. We also add a small helper, so that we don't hardcode the
RSUnavail case in many places.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>