KVM: selftests: Use a loop to create guest page tables

KVM: selftests: Use a loop to create guest page tables

Walk the guest page tables via a loop when creating new mappings,
instead of using unique variables for each level of the page tables.

This simplifies the code and makes it easier to support 5-level paging
in the future.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251028225827.2269128-2-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>