ui: ProcessorEdit: allow modifications with inaccessible CPU model

ui: ProcessorEdit: allow modifications with inaccessible CPU model

An administrator can set a custom CPU model for a VM where the general user
does not have permission to use this particular model. Prior to this change
the ProcessorEdit component would be broken by this, since the store of the
CPU type selector did not contain the configured CPU model.

Add it in manually if this situation occurs (with 'Unknown' vendor, since
we cannot retrieve it from the API), but warn the user that changing it
would be an irreversible action.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>