[HVM][VMX] Clear vmxe when VMX is not enabled.

[HVM][VMX] Clear vmxe when VMX is not enabled.

The current Xen code keeps X86_CR4_VMXE set even if VMXON has not been
executed. The stop_vmx() code assumes that it is possible to call
VMXOFF if X86_CR4_VMXE is set which is not always true. Calling VMXOFF
without VMXON results in an illegal opcode trap, and to avoid this condition
this patch makes sure that X86_CR4_VMXE is only set when VMXON has
been called.

Tested using x86_32 on a Pentium D 930.

Signed-Off-By: Magnus Damm <magnus@valinux.co.jp>