KVM: VMX: Disable VMX when system shutdown

Intel TXT(Trusted Execution Technology) required VMX off for all cpu to work
when system shutdown.

CC: Joseph Cihula <joseph.cihula@intel.com>
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>