target/ppc: Fold slb_nr into PPCHash64Options

target/ppc: Fold slb_nr into PPCHash64Options

The env->slb_nr field gives the size of the SLB (Segment Lookaside Buffer).
This is another static-after-initialization parameter of the specific
version of the 64-bit hash MMU in the CPU.  So, this patch folds the field
into PPCHash64Options with the other hash MMU options.

This is a bit more complicated that the things previously put in there,
because slb_nr was foolishly included in the migration stream.  So we need
some of the usual dance to handle backwards compatible migration.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>