KVM: arm64: Expose support for private memory

Select KVM_GENERIC_PRIVATE_MEM and provide the necessary support
functions.

Signed-off-by: Steven Price <steven.price@arm.com>
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Changes since v2:
 * Switch kvm_arch_has_private_mem() to a macro to avoid overhead of a
   function call.
 * Guard definitions of kvm_arch_{pre,post}_set_memory_attributes() with
   #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES.
 * Early out in kvm_arch_post_set_memory_attributes() if the WARN_ON
   should trigger.