x86/access: Use upper half of virtual address space

Use the upper half of the virtual address space so that 5-level paging
doesn't collide with the core infrastucture in the top-level PTE, which
hides bugs, e.g. SMEP + 5-level, and is generally a nightmare to debug.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>