KVM fixes for 6.10

 - Fix a "shift too big" goof in the KVM_SEV_INIT2 selftest.

 - Compute the max mappable gfn for KVM selftests on x86 using GuestMaxPhyAddr
   from KVM's supported CPUID (if it's available).

 - Fix a race in kvm_vcpu_on_spin() by ensuring loads and stores are atomic.

 - Fix technically benign bug in __kvm_handle_hva_range() where KVM consumes
   the return from a void-returning function as if it were a boolean.