KVM: selftests: Add module param API to check if nested virtualization is enabled

Add an API to check if the nested virtualization module param is enabled,
e.g. so that tests can verify KVM's own enumeration of the corresponding
CPU feature.  Provide a big warning above the API to make it as obvious as
possible clear that tests should use "kvm_cpu_has(X86_FEATURE_VMX) ||
kvm_cpu_has(X86_FEATURE_SVM)" when checking for nested virtualization.  It
doesn't really matter which method tests use, as KVM is supposed to keep
"nested" synchronized with VMX/SVM enumeration, but make the comment super
scary to try and prevent KVM selftests from having two different ways of
doing the same thing.

Note, the bool vs. int mess is intentional, because KVM's params are
terrible.  kvm_amd.ko provides an "int", while kvm_intel.ko provides a
"bool".

Link: https://patch.msgid.link/20260930212114.3500045-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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