KVM: arm64: Move 'struct kvm_arch_memory_slot' out of uapi/

'struct kvm_arch_memory_slot' isn't part of the user ABI, so move it out
of the uapi/ headers in case we start using it in future and accidentally
back ourselves into a corner.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201118194402.2892-2-will@kernel.org