Comment 44 for bug 1393842

In comparison, when attempting to lunch a VitualBox VM, it fails with a slightly better error message at least directing one to investigate what else would be using a hypervisor. But it also suggests some rather drastic steps to do with recompiling the kernel to remove KVM kernel extension - wow:

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Failed to open a session for the virtual machine core-plus-vm.

VT-x is being used by another hypervisor (VERR_VMX_IN_VMX_ROOT_MODE).

VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot (VERR_VMX_IN_VMX_ROOT_MODE).

Result Code: NS_ERROR_FAILURE (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}