KVM: selftests: Use virt_get_pte() when getting PTE pointer

Use virt_get_pte() in vm_get_page_table_entry() instead of open coding
equivalent code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006004512.666529-6-seanjc@google.com