Support default images for image mode in virtual provisioner

[thrix-bot]

When using image mode with the virtual provisioner, users currently need to specify a full image URL. It would be useful to support a simpler interface where the user can enable image mode and have tmt automatically pick the image mode variant of whatever default/short-name image is being used.

For example, instead of requiring a full qcow2 URL, users could write:

provision:
    how: virtual
    image_mode: true

and tmt would translate the default image (e.g., fedora-rawhide) to its image mode variant automatically.

This would make it easier for users to opt into image mode without needing to know the exact image URLs, similar to how short-name images already work for regular virtual provisioning.

---

Origin: teemtee/tmt#4719 (comment) (suggestion by @LecrisUT)

[happz]

From the triage meeting: seems like a job for testcloud package. Define "flavor" of images, e.g. /image-mode, and when image: fedora/image-mode is used, testcloud picks the right image. Maybe testcloud already supports similar extra hints.