Resolve "Transform Virtual Machine Service Offering to linkml"

Closes #262 (closed)

Merge after MR !189 (merged).

Tests to write:

• Quantity ( @anja.strunk)
• Device ( @mpetruzzella1)
• GPU ( @anja.strunk)
• CPU ( @anja.strunk)
• Disk ( @mpetruzzella1)
• Memory ( @mpetruzzella1)
• Image ( @anja.strunk)
• VM Image ( @anja.strunk)
• PXE Image ( @anja.strunk)
• Server Flavor (@mpetruzzella1)
• Code Artifact ( abstract class without any attributes --> no tests required)
• Installation Requirement ( abstract class without any attributes --> no tests required)
• Software Resource ( @anja.strunk)
• Operation System ( @anja.strunk)
• Hypervisor ( @anja.strunk)
• Cryptography ( @anja.strunk)
• Infrastructure Service Offering ( abstract class without any attributes --> no tests required)
• Compute Service Offering ( @anja.strunk)
• Virtual Machine Service Offering ( @anja.strunk)