KVM: selftests: Remove useless shifts when creating guest page tables

Remove the pointless shift from GPA=>GFN and immediately back to
GFN=>GPA when creating guest page tables.  Ignore the other walkers
that have a similar pattern for the moment, they will be converted
to use virt_get_pte() in the near future.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006004512.666529-4-seanjc@google.com