KVM: arm64: nv: Handle VNCR_EL2-triggered faults backed by guest_memfd

KVM: arm64: nv: Handle VNCR_EL2-triggered faults backed by guest_memfd

Handle faults for memslots backed by guest_memfd in arm64 nested
virtualization triggered by VNCR_EL2.

* Introduce is_gmem output parameter to kvm_translate_vncr(), indicating
  whether the faulted memory slot is backed by guest_memfd.

* Dispatch faults backed by guest_memfd to kvm_gmem_get_pfn().

* Update kvm_handle_vncr_abort() to handle potential guest_memfd errors.
  Some of the guest_memfd errors need to be handled by userspace instead
  of attempting to (implicitly) retry by returning to the guest.

Suggested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-20-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>