KVM: selftests: Add __open_path_or_exit() variant to provide extra help info

KVM: selftests: Add __open_path_or_exit() variant to provide extra help info

Add an inner __open_path_or_exit() API to let the caller provide additional
information on ENOENT to try and help the user figure out why the test is
being skipped, e.g. for files like the page_idle bitmap needed by the
access tracking perf, which is dependent on a Kconfig.

Immediately convert /dev/kvm to the new API, both as an example and because
users might not know that some architectures/setups require loading KVM.

Link: https://lore.kernel.org/r/20250516215909.2551628-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>