UTM 5.03 eats all memory

**Describe the issue**
I'm running UTM 5.03 (Beta) on macOS Tahoe 26.5.1 and used a Windows 11 ARM that has not been stopped for a log time (at least a week). So the VM was on for this time. This morning, macOS displayed a message about the system is running out of memory.

In fact, UTM was hooking about 98GB on a Macbook Pro with 16GB physical RAM. Stopping the Windows VM freed about 2 GB but the UTM backend was still using 96 GB.

Recovering the memory was only possible by closing UTM

The Windows 11 ARM is 25H2 with the latest patches (June 2026)

**Configuration**
* UTM Version: 5.0.3 (Beta)
* macOS Version: macOS Tahoe 26.5.1
* Mac Chip (Intel, M1, ...): Apple Silicon M1 Pro

**Crash log**
No Crash log

**Debug log**

[debug-memory usage.log.zip](https://github.com/user-attachments/files/29404420/debug-memory.usage.log.zip)

Start by reviewing debug-memory usage.log.zip and reproducing the report with UTM 5.0.3 on an Apple Silicon Mac using a long-running Windows 11 ARM VM. Compare memory usage while the VM runs, after stopping it, and after closing UTM; done means the backend no longer grows to extreme levels and memory is reclaimed when the VM stops.

Written by the indexing model from the issue text.