KVM: selftests: Add another underscore to inner ioctl() helpers

Add a second underscore to inner ioctl() helpers to better align with
commonly accepted kernel coding style, and to allow using a single
underscore variant in the future for macro shenanigans.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>