Allow rpc.MakeLegacyNodeInfo to parse non-LVM results

'MakeLegacyNodeInfo' is not the best place for this, but we'd have to
duplicate it if we wanted a LVM-less version, so the easiest is to add
an optional parameter that allows it to accept/skip LVM-less results.

It still requires at most one VG result, so its behaviour isn't
changed in this respect.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>