KVM: nVMX: fix lifetime issues for vmcs02

KVM: nVMX: fix lifetime issues for vmcs02

free_nested needs the loaded_vmcs to be valid if it is a vmcs02, in
order to detach it from the shadow vmcs.  However, this is not
available anymore after commit a1907ee70545 (KVM: VMX: fix use after
free of vmx->loaded_vmcs, 2014-01-03).

Revert that patch, and fix its problem by forcing a vmcs01 as the
active VMCS before freeing all the nested VMX state.

Reported-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Tested-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>