KVM: VMX: Clear CR4.VMXE in hardware_disable

Clear CR4.VMXE in hardware_disable. There's no reason to leave it set
after doing a VMXOFF.

VMware Workstation 6.5 checks CR4.VMXE as a proxy for whether the CPU is
in VMX mode, so leaving VMXE set means we'll refuse to power on. With this
change the user can power on after unloading the kvm-intel module. I
tested on kvm-67 and kvm-69.

Signed-off-by: Eli Collins <ecollins@vmware.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>