VMs located externally become unavailable when UTM is reopened

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**Describe the issue**
Virtual machines placed externally (such as on an external drive) have to be "re-added" to UTM every time the app is reopened. Doesn't seem to matter what type of VM it is.
I have tried giving UTM Full Disk Access perms as well, but it didn't make difference.

Example pic below has 3 VMs, first one is a virtualized macOS VM located externally, second is an emulated VM located internally in UTM, and last one is emulated VM located externally. The externally located ones become unavailable every time UTM is reopened, and have to be re-added by double-clicking the .utm file in Finder.

**Configuration**
* UTM Version: 4.7.5 (118)
* macOS Version: 26.5.1 (25F80)
* Mac Chip (Intel, M1, ...): M4 (10 core)

Reproduce the issue by adding a .utm virtual machine from an external drive, quitting UTM, and reopening it. Trace the macOS VM list and persistence behavior around relaunch; done means externally located VMs remain available after reopening without being re-added.

Written by the indexing model from the issue text.