Switch to the vm_vaddr_alloc_page() helper for x86-64's "kernel"
allocations now that the helper uses the same min virtual address as the
open coded versions.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>