When running in a virtualized environment, TLB invalidations can only
be performed on process scope, as only the hypervisor is allowed to
invalidate a global scope, or else a Program Interrupt is triggered.

Since we are here, also make sure that the register process table
hypercall returns success.

Tested on QEMU with KVM enabled, on a POWER9 machine.
(currently depends on a patched loader version to work, to call CAS and select Radix with GTSE)