spapr: Simplify ovec diff

spapr: Simplify ovec diff

spapr_ovec_diff(ov, old, new) has somewhat complex semantics.  ov is set
to those bits which are in new but not old, and it returns as a boolean
whether or not there are any bits in old but not new.

It turns out that both callers only care about the second, not the first.
This is basically equivalent to a bitmap subset operation, which is easier
to understand and implement.  So replace spapr_ovec_diff() with
spapr_ovec_subset().

Cc: Mike Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cedric Le Goater <clg@fr.ibm.com>