KVM: selftests: Add hugepage support for x86-64

Add x86-64 hugepage support in the form of a x86-only variant of
virt_pg_map() that takes an explicit page size.  To keep things simple,
follow the existing logic for 4k pages and disallow creating a hugepage
if the upper-level entry is present, even if the desired pfn matches.

Opportunistically fix a double "beyond beyond" reported by checkpatch.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-19-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>