Solaris kernel zones on x86-based systems make use of hardware-assisted virtualization features like VirtualBox does. However, for kernel zones and VirtualBox to share this hardware resource, they need to co-operate.
By default, due to performance reasons, VirtualBox acquires the hardware-assisted virtualization resource (VT-x/AMD-V) globally on the host machine and uses it until the last VirtualBox VM that requires it is powered off. This prevents other software from using VT-x/AMD-V during the time VirtualBox has taken control of it.
VirtualBox can be instructed to relinquish use of hardware-assisted virtualization features when not executing guest code, thereby allowing kernel zones to make use of them. To do this, shutdown all VirtualBox VMs and execute the following command:
VBoxManage setproperty hwvirtexclusive off
This command needs to be executed only once as the setting is stored as part of the global VirtualBox settings which will continue to persist across host-reboots and VirtualBox upgrades.