performance-metrics: Switch to QcowDisk

Switch from QcowFile to QcowDisk taking advantage of QcowTempDisk where
appropriate.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Rob Bradford <rbradford@meta.com>

block/Cargo.toml

@@ -8,6 +8,7 @@ version = "0.1.0"
 [features]
 default = []
 io_uring = ["dep:io-uring"]
+test-utils = []
 
 [dependencies]
 bitflags = { workspace = true }