accel/kvm: Avoid Coverity warning in query_stats()

Coverity complains that there is a codepath in the query_stats()
function where it can leak the memory pointed to by stats_list.  This
can only happen if the caller passes something other than
STATS_TARGET_VM or STATS_TARGET_VCPU as the 'target', which no
callsite does.  Enforce this assumption using g_assert_not_reached(),
so that if we have a future bug we hit the assert rather than
silently leaking memory.

Resolves: Coverity CID 1490140
Fixes: cc01a3f4ca ("kvm: Support for querying fd-based stats")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220719134853.327059-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -4014,7 +4014,7 @@ static void query_stats(StatsResultList **result, StatsTarget target,
                         stats_list);
         break;
     default:
-        break;
+        g_assert_not_reached();
     }
 }