KVM: VMX: fix detection of BIOS disabling VMX

This patch fixes the logic used to detect whether BIOS has disabled VMX, for
the case where VMX is enabled only under SMX, but tboot is not active.

Signed-off-by: Joseph Cihula <joseph.cihula@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>