KVM: x86/mmu: Propagate memslot const qualifier

In preparation for implementing in-place hugepage promotion, various
functions will need to be called from zap_collapsible_spte_range, which
has the const qualifier on its memslot argument. Propagate the const
qualifier to the various functions which will be needed. This just serves
to simplify the following patch.

No functional change intended.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20211115234603.2908381-11-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>