KVM: VMX: Fix a wrong usage of vmcs_config

The function ept_update_paging_mode_cr0() write to
CPU_BASED_VM_EXEC_CONTROL based on vmcs_config.cpu_based_exec_ctrl. That's
wrong because the variable may not consistent with the content in the
CPU_BASE_VM_EXEC_CONTROL MSR.

Signed-off-by: Sheng Yang <sheng.yang@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>