VM names should not allow keywords

[svenssonaxel]

Qubes OS version (e.g., R3.2):

R3.2

Affected TemplateVMs (e.g., fedora-23, if applicable):

N/A

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Steps to reproduce the behavior:

Use the VM Manager to create a new NetVM with the name "default".

Expected behavior:

The operation fails, similarly to the names "none", "true" and "false".

Actual behavior:

The NetVM is created. This leads to an ambiguity in the following command:
qvm-prefs -s my-new-vm netvm default

General notes:

To avoid similar mistakes in the future, maybe keywords should be uniformly distinguished from VM names, e.g. begin with _ or $, similarly to the current policy file format that uses $any and $dispvm. This would also provide backward compatibility with backups if new keywords are introduced in the future.

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Related issues:

[marmarek]

Fixed in QubesOS/updates-status#293

[marmarek]

Fixed in QubesOS/updates-status#293