KVM: selftests: Isolate the guest_memfd Copy-on-Write negative testcase

Move the guest_memfd Copy-on-Write (CoW) testcase to its own function to
better separate positive testcases from negative testcases.

No functional change intended.

Suggested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-11-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>