ANDROID: KVM: arm64: Use nr_pages for host_unshare_guest()

__pkvm_host_unshare_guest() had grown an 'order' argument to match the
existing Android code. But it is now the only function left with that
argument, and is inconsistent with host_{share,donate}_guest() which
take 'nr_pages' instead. Align the unshare path by using 'nr_pages' as
well.

Bug: 357781595
Bug: 235345873
Change-Id: I8de9f435dab738ece370c6d76c72932bfed38030
Signed-off-by: Quentin Perret <qperret@google.com>