x86/access: Rename variables in page table walkers

Rename variables in the page table walkers to get rid of the awful root
and vroot terminology, which is obsolete/wrong for everything except the
top-level entry.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>