pnv/xive: Fix possible undefined shift error in group size calculation

pnv/xive: Fix possible undefined shift error in group size calculation

Coverity discovered a potential shift overflow in group size calculation
in the case of a guest error. Add checks and logs to ensure a issues are
caught.

Make the group and crowd error checking code more similar to one another
while here.

Resolves: Coverity CID 1593724
Fixes: 9cb7f6ebed60 ("ppc/xive2: Support group-matching when looking for target")
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>