KVM: arm64: placeholder to check if VM is protected

Add a function to check whether a VM is protected (under pKVM).
Since the creation of protected VMs isn't enabled yet, this is a
placeholder that always returns false. The intention is for this
to become a check for protected VMs in the future (see Will's RFC).

No functional change intended.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/kvmarm/20210603183347.1695-1-will@kernel.org/
Link: https://lore.kernel.org/r/20210817081134.2918285-2-tabba@google.com