KVM: arm64: Rename pkvm.enabled to pkvm.is_protected

KVM: arm64: Rename pkvm.enabled to pkvm.is_protected

The 'pkvm.enabled' field in struct kvm_protected_vm is confusingly
named. Its purpose is to indicate whether a VM is a _protected_ VM under
pKVM, and not whether the VM itself is enabled or running.

For a non-protected VM, the VM can be fully active and running, yet this
field would be false. This ambiguity can lead to incorrect assumptions
about the VM's operational state and makes the code harder to reason
about.

Rename the field to 'is_protected' to make it unambiguous that the flag
tracks the protected status of the VM.

No functional change intended.

Reviewed-by: Kunwu Chan <kunwu.chan@linux.dev>
Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Kunwu Chan <chentao@kylinos.cn>
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>