When NX is enabled on the host but not on the guest, we use the entry/exit
msr load facility, which is slow.  Optimize it to use entry/exit efer load,
which is ~1200 cycles faster.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>