snapshot delete gives error after disk type change from normal to immutable

[Smackey's Dad]

Steps to reproduce:

• VM has one snapshot
• Change attached disk type from normal to immutable
• After change (which goes through) deleting the (one) snapshot gives an error

VBoxManage snapshot "${VB_VM_NAME}" delete snapshot1
0%...
Progress state: NS_ERROR_FAILURE
VBoxManage: error: Snapshot operation failed
VBoxManage: error: Hard disk '/root/avm/vms/Trusty/Snapshots/{b8b3a8e8-1151-4f62-b0d0-bf538e509eed}.vdi' has more than one child hard disk (2)
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component SessionMachine, interface IMachine
VBoxManage: error: Context: "int handleSnapshot(HandlerArg*)" at line 431 of file VBoxManageSnapshot.cpp

[aeichner]

Please reopen if still relevant with a recent VirtualBox release.