performance-metrics: Add QCOW2 backing file read micro benchmark

Add micro_bench_qcow_backing_read which reads clusters from a QCOW2
overlay where all data lives in a raw backing file.  Every read falls
through the L2 lookup to the backing file, exercising the backing
chain read path.

Workloads: 128 and 256 clusters.
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

performance-metrics/src/main.rs

@@ -378,7 +378,7 @@ mod adjuster {
     }
 }
 
-const TEST_LIST: [PerformanceTest; 72] = [
+const TEST_LIST: [PerformanceTest; 74] = [
     PerformanceTest {
         name: "boot_time_ms",
         func_ptr: performance_boot_time,
@@ -1373,6 +1373,30 @@ const TEST_LIST: [PerformanceTest; 72] = [
         },
         unit_adjuster: adjuster::s_to_us,
     },
+    PerformanceTest {
+        name: "micro_block_qcow_backing_read_128_us",
+        func_ptr: micro_bench_block::micro_bench_qcow_backing_read,
+        control: PerformanceTestControl {
+            test_timeout: 10,
+            test_iterations: 20,
+            warmup_iterations: 5,
+            num_ops: Some(128),
+            ..PerformanceTestControl::default()
+        },
+        unit_adjuster: adjuster::s_to_us,
+    },
+    PerformanceTest {
+        name: "micro_block_qcow_backing_read_256_us",
+        func_ptr: micro_bench_block::micro_bench_qcow_backing_read,
+        control: PerformanceTestControl {
+            test_timeout: 10,
+            test_iterations: 20,
+            warmup_iterations: 5,
+            num_ops: Some(256),
+            ..PerformanceTestControl::default()
+        },
+        unit_adjuster: adjuster::s_to_us,
+    },
 ];
 
 fn run_test_with_timeout(