WIP: Feature/kvm drop default image
Tasks:
- 
thread [kvm] Dropping default image  - 
it's rebased over !1667 (merged) and most possibly shall be merge as one (closing the other)  - 
behavior with virtual-hard-drive-urlandvirtual-hard-drive-md5sum- default shall not be used, select will be used
 
 - 
behavior with nbd-host:nbd-portandnbd2-host:nbd2-port- default shall not be used, select will be used
 
 - 
depends !1571 (merged)  - 
BLOCKER: inconsistency in SlapOS Master JSON UI with handling non existing elements on the lists has to be solved  - 
!1174 (merged)  - 
!1176 (merged)  - 
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
 
 
 - watch out for backward compatibility!
 - 
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-selecthas precedence with default - 
boot-image-url-listdisables theboot-image-url-selectdefault - 
NBD ( nbd-host,nbd-port,nbd2-host,nbd2-port) disables theboot-image-url-selectdefault - 
virtual-hard-drive-urlpresence disables theboot-image-url-selectdefault