KVM: selftests: Assert that vcpu->cpuid is non-NULL when getting CPUID entries

Add a sanity check in __vcpu_get_cpuid_entry() to provide a friendlier
error than a segfault when a test developer tries to use a vCPU CPUID
helper on a barebones vCPU.

Link: https://lore.kernel.org/r/20241128013424.4096668-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/include/x86/processor.h

@@ -1014,6 +1014,8 @@ static inline struct kvm_cpuid_entry2 *__vcpu_get_cpuid_entry(struct kvm_vcpu *v
 							      uint32_t function,
 							      uint32_t index)
 {
+	TEST_ASSERT(vcpu->cpuid, "Must do vcpu_init_cpuid() first (or equivalent)");
+
 	return (struct kvm_cpuid_entry2 *)get_cpuid_entry(vcpu->cpuid,
 							  function, index);
 }