Oracle VM Server: my first vm: Error: HVM guest support is unavailable

All my tests with Oracle VM Server are running inside Oracle Virtualbox. If you want to do some tests yourself with this setup, you can easily get this error message after powering on your VM:

Server error: Command: ['xm', 'create', '/OVS/Repositories/0004fb0000030000dad74d9c43176d2e/VirtualMachines/0004fb00000600005e79798ecb1a63cf/vm.cfg'] failed (1): stderr: Error: HVM guest support is unavailable: is VT/AMD-V supported by your CPU and enabled in your BIOS?
stdout: Using config file "/OVS/Repositories/0004fb0000030000dad74d9c43176d2e/VirtualMachines/0004fb00000600005e79798ecb1a63cf/vm.cfg".

To get your system running, you have to change this for your Oracle VM Server node on VirtualBox:

After that you get:

Server error: Command: ['xm', 'create', '/OVS/Repositories/0004fb0000030000dad74d9c43176d2e/VirtualMachines/0004fb0000060000b5dca8dccb8b74f6/vm.cfg'] failed (1): stderr: Error: Boot loader didn't return any data!
stdout: Using config file "/OVS/Repositories/0004fb0000030000dad74d9c43176d2e/VirtualMachines/0004fb0000060000b5dca8dccb8b74f6/vm.cfg".

But this is due to specifying no boot media at your VM. Therefore you have to add an ISO image to your Oracle VM Server repository.