[FIX] calendar: fetching images of negative ids

The mimetime fetching in /web/image/... may fail if:
- calendar is installed
- `id` is a string (using ?id=42 in the URI)
- `id` is a negative number (company partners on odoo.com)

The `search` override of `ir.attachment` in calendar detects if a number is a
virtual ID when it contains a '-' and is a string.

While it is not optimal to allow both int and str format for paramter `id`, make
the verification of virtual ID in calendar more strict to only match the format
`<real>-<virtual>` and not `-<real>`