NodeQuery: mark live fields as UNAVAIL for non-vm_capable nodes

NodeQuery: mark live fields as UNAVAIL for non-vm_capable nodes

Since we don't have the data per design, UNAVAIL is appropriate here,
while NODATA is not.

The patch also adds a comment: if we extend the live fields list to
contain other data in the future, we need to reevaluate this solution.

This should fix issue 143. The listing now shows (node2==ofline,
node3==not vm_capable):

  Node     DTotal     DFree    MTotal     MNode     MFree Pinst Sinst
  node1    698.6G    630.5G     32.0G      1.0G     30.0G     8     7
  node2 (offline) (offline) (offline) (offline) (offline)     9     4
  node3 (unavail) (unavail) (unavail) (unavail) (unavail)     0     0

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>