The HAXM kext appears to grab the VT-X extensions and refuses to release it when unloaded. This means that Virtualbox fails when attmempting to run a guest OS. Virtualbox stops working when HAXM is installed and does not work again until the extension is unloaded and the machine is rebooted. While HAXM seems like an excellent solution to speed up Android emulation (although I have yet to acutally try it), I need Virtualbox more.
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