Add model documentation for VM interfaces

docs/core-functionality/virtualization.md

@@ -7,3 +7,4 @@
 ---
 
 {!docs/models/virtualization/virtualmachine.md!}
+{!docs/models/virtualization/vminterface.md!}

docs/models/virtualization/vminterface.md

@@ -0,0 +1,3 @@
+## Interfaces
+
+Virtual machine interfaces are similar to device interfaces, but lack properties pertaining to physical attributes. For example, VM interfaces do not have a physical type and cannot have cables attached to them. However, they can have IP address and VLANs (both tagged and untagged) associated with them, just as device interfaces do.