Whilst working on getting a VM to initialize in to IA32e mode I found
this issue. set_cr0 relies on comparing the old cr0 to the new one to
work correctly.  Move the assignment below so the compare can work.
Signed-off-by: NPaul Knowles <paul@transitive.com>
Signed-off-by: NAvi Kivity <avi@qumranet.com>