api: import-metadata: make warnings structured & merge ignored-volumes

api: import-metadata: make warnings structured & merge ignored-volumes

This allows the frontends to translate them and avoids somewhat
duplicated info by having some warnings explicitly (ignored-volumes)
while others are in the warnings array.

By passing along the key and the value the frontend can also show the
warnings in-line, e.g. by marking a disk-entry in a grid as having
potential problems.

Ideally we'd have a central list of known types used for the API
return schema enum and to check when calling the $warn closure, but as
we only got three warnings keep this as is and only add a comment.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>