KVM: selftests: Fix nsec to sec conversion in demand_paging_test

KVM: selftests: Fix nsec to sec conversion in demand_paging_test

demand_paging_test uses 1E8 as the denominator to convert nanoseconds to
seconds, which is wrong. Use NSEC_PER_SEC instead to fix the issue and
make the conversion obvious.

Reported-by: James Houghton <jthoughton@google.com>
Signed-off-by: Anish Moorthy <amoorthy@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230223001805.2971237-1-amoorthy@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>