run: fix GetList return empty issue for throttledevice

Test "--device-read-bps" "--device-write-bps" will fail. The root
cause is that GetList helper return empty as its local variable
initialized to zero size.

This patch fix it by setting the related slice size to non-zero.

Signed-off-by: Jianyong Wu <wujianyong@hygon.cn>
Fixes: #5321

opts/throttledevice.go

@@ -94,7 +94,7 @@ func (opt *ThrottledeviceOpt) String() string {
 
 // GetList returns a slice of pointers to ThrottleDevices.
 func (opt *ThrottledeviceOpt) GetList() []*blkiodev.ThrottleDevice {
-	out := make([]*blkiodev.ThrottleDevice, 0, len(opt.values))
+	out := make([]*blkiodev.ThrottleDevice, len(opt.values))
 	copy(out, opt.values)
 	return out
 }