conf: Don't format cputune element when not needed

conf: Don't format cputune element when not needed

Commit 60b176c3d0f0d5037acfa5e27c7753f657833a0b introduced a bug that
when editing an XML with cputune similar to this:

...
  <vcpu placement='static' current='1'>2</vcpu>
  <cputune>
    <vcpupin vcpu="1" cpuset="0"/>
  </cputune>
...

results in formatted XML that looks like this:

...
  <vcpu placement='static' current='1'>2</vcpu>
  <cputune>
  </cputune>
...

That is caused by a condition depending on def->cputune.vcpupin being
set rather than checking def->cputune.nvcpupin.  Notice that nvcpupin
can be 0 and vcpupin can still be allocated since it's a pointer to an
array, so no harm done there.

I also changed it on other places in the code where it depended on the
wrong variable.