Can it be, that using -F with ovm_vmcrontrol is what you'd need to do?
[root@oraclevmm ~]# ovm_vmcontrol -h
Oracle VM VM Control utility 0.6.4.
The Oracle VM VM Control utility allows the user to send
simple control commands to a Virtual Machine
-u [user name] Oracle VM Manager admin username (required)
-p [password] admin password (required)
-h [hostname] Oracle VM Manager hostname (required)
-X use secure (ssl) connection to port 54322
-v [vm name] virtual machine name
-U [vm UUID] virtual machine UUID
-c [command] console command (required)
[start, stop, suspend, resume, status, restart, kill, info, lock, list, migrate, events, vcpuset, vcpuget, gettags, settags, fixcfg, delete, setoncrash, getoncrash]
-t [target] target server name (in conjunction with -c migrate)
-s [cpulist] comma separated cpu thread list(in conjunction with -c vcpuset)
-T [taglist] comma separated tags list (in conjunction with -c settags
-A acknowledge events (in conjunction with -c events
-F force delete virtual disks (in conjunction with -c delete
-C on_crash setting : [preserve, restart] (in conjunction with -c getoncrash)
So, a ovm_vmcontrol -c delete -F .... should remove the associated vdisks as well.