The bochs BIOS does not set the LOCK_BIT of MSR_FEATURE_CONTROL when VMX is enabled.
I found the following workaround:
cpu/init.cc line 1100 has a comment that VMX should be enabled by the BIOS instead.
When uncommenting msr.ia32_feature_control = BX_IA32_FEATURE_LOCK_BIT...
everything works fine.
Bug was found when trying to boot NOVA (www.hypervisor.org). NOVA reported that the CPU
does not support VMX even when compiled with --enable-vmx=2 and cpu set to sandybridge 2600k.