KVM: arm64: Add a range to __pkvm_host_unshare_guest()

In preparation for supporting stage-2 huge mappings for np-guest. Add a
nr_pages argument to the __pkvm_host_unshare_guest hypercall. This range
supports only two values: 1 or PMD_SIZE / PAGE_SIZE (that is 512 on a
4K-pages system).

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://lore.kernel.org/r/20250521124834.1070650-5-vdonnefort@google.com


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