spapr: nested: register nested-hv api hcalls only for cap-nested-hv

spapr: nested: register nested-hv api hcalls only for cap-nested-hv

Since cap-nested-hv is an optional capability, it makes sense to register
api specfic hcalls only when respective capability is enabled. This
requires to introduce a new API to unregister hypercalls to maintain
sanity across guest reboot since caps are re-applied across reboots and
re-registeration of hypercalls would hit assert otherwise.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>