KVM: arm64: Introduce __pkvm_host_unshare_guest()

In preparation for letting the host unmap pages from non-protected
guests, introduce a new hypercall implementing the host-unshare-guest
transition.

Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20241218194059.3670226-12-qperret@google.com


Signed-off-by: Marc Zyngier <maz@kernel.org>