KVM: selftests: Fail tests when open() fails with !ENOENT

KVM: selftests: Fail tests when open() fails with !ENOENT

open_path_or_exit() is used for '/dev/kvm', '/dev/sev', and
'/sys/module/%s/parameters/%s' and skipping test when the entry is missing
is completely reasonable. Other errors, however, may indicate a real issue
which is easy to miss. E.g. when 'hyperv_features' test was entering an
infinite loop the output was:

./hyperv_features
Testing access to Hyper-V specific MSRs
1..0 # SKIP - /dev/kvm not available (errno: 24)

and this can easily get overlooked.

Keep ENOENT case 'special' for skipping tests and fail when open() results
in any other errno.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240129085847.2674082-2-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>