[catborise]

Closes #175.

The create form and the API accepted 0 or a negative number for vCPUs, memory and a new disk's size:

• The API queued the job (202), which then failed later in libvirt. The test reproduced the 202 before the fix.
• The page did the same. It posts with formnovalidate, so the browser's min=1 was skipped.

The fix

• The CreateInstance model's vcpu, memory and hdd_size fields refuse values below 1 (MinValueValidator(1)). Both the form and the API serializer are built on this model, so both check it. No migration is needed (makemigrations --check is clean).
• NewVMForm checks the MAC addresses with nic_macs, as the API and the job already do. A bad MAC is now an error on the form instead of a job that fails.

Verification

• New tests, red before the fix and green after it:
  • the page shows each size error beside its field and queues nothing;
  • a bad MAC is a form error;
  • the API answers 400 naming the field and queues nothing.
• The full suite passes on SQLite and on PostgreSQL 16: 1391 tests each, OK (32 skipped).

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