WIP: Feature/kvm drop default image

Tasks:

• thread [kvm] Dropping default image
• it's rebased over https://lab.nexedi.com/nexedi/slapos/-/merge_requests/1667 and most possibly shall be merge as one (closing the other)
• behavior with virtual-hard-drive-url and virtual-hard-drive-md5sum
  • default shall not be used, select will be used
• behavior with nbd-host:nbd-port and nbd2-host:nbd2-port
  • default shall not be used, select will be used
• depends https://lab.nexedi.com/nexedi/slapos/-/merge_requests/1571
• BLOCKER: inconsistency in SlapOS Master JSON UI with handling non existing elements on the lists has to be solved
• https://lab.nexedi.com/nexedi/slapos/merge_requests/1174
• https://lab.nexedi.com/nexedi/slapos/merge_requests/1176
• drop the image URLs from the schema and put them into profiles
  • watch out for backward compatibility!
    • compatibility solved by failing the instance until proper value is selected
• the default shall be Debian Bullseye 12 netinst x86_64, so that microchanges of the release won't be so important

Logic:

• boot-image-url-select has precedence with default
• boot-image-url-list disables the boot-image-url-select default
• NBD (nbd-host, nbd-port, nbd2-host, nbd2-port) disables the boot-image-url-select default
• virtual-hard-drive-url presence disables the boot-image-url-select default