fix(blade): ask only the blades of a chassis for a discovery inventory

The findme handler of the blade plugin makes an inventory request for each
node in the mp table. That table holds more than the blades of a chassis.
lsslp writes a row for a Power BMC, for an FSP, for a BPA, for an HMC and for
other hardware, and xCAT::PPCdb::add_systemX writes a row for a management
module. None of that hardware answers a blade inventory.

Keep a row that gives blade as its hardware type. Keep also a row that gives
no hardware type but names a different node as its mpa, when that other node
is a management module. The mp template in xCAT/templates/e1350 leaves the
hardware type of a blade empty, so a test of the hardware type alone loses
the blades of a chassis.

Return when the table holds no blades. The work below the filter reads the
arp table of the management node, and a site that has no chassis must not pay
for that on each discovery request.

Recovered from the lenovobuild branch, which tested the hardware type only.