Add a helper to allocate a page for use in constructing the guest's page
tables.  All architectures have identical address and memslot
requirements (which appear to be arbitrary anyways).

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>