KVM: x86: Refactor CPUID 0xD.N sub-leaf entry creation

Increment the number of CPUID entries immediately after do_host_cpuid()
in preparation for moving the logic into do_host_cpuid().  Handle the
rare/impossible case of encountering a bogus sub-leaf by decrementing
the number entries on failure.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -677,6 +677,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 				goto out;
 
 			do_host_cpuid(&entry[i], function, idx);
+			++*nent;
 
 			/*
 			 * The @supported check above should have filtered out
@@ -685,12 +686,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
 			 * reach this point, and they should have a non-zero
 			 * save state size.
 			 */
-			if (WARN_ON_ONCE(!entry[i].eax || (entry[i].ecx & 1)))
+			if (WARN_ON_ONCE(!entry[i].eax || (entry[i].ecx & 1))) {
+				--*nent;
 				continue;
+			}
 
 			entry[i].ecx = 0;
 			entry[i].edx = 0;
-			++*nent;
 			++i;
 		}
 		break;