conf: Don't mangle vcpu placement randomly

conf: Don't mangle vcpu placement randomly

https://bugzilla.redhat.com/show_bug.cgi?id=1170492

In one of our previous commits (dc8b7ce7) we've done a functional
change even though it was intended as pure refactor. The problem is,
that the following XML:

 <vcpu placement='static' current='2'>6</vcpu>
 <cputune>
   <emulatorpin cpuset='1-3'/>
 </cputune>
 <numatune>
   <memory mode='strict' placement='auto'/>
 </numatune>

gets translated into this one:

 <vcpu placement='auto' current='2'>6</vcpu>
 <cputune>
   <emulatorpin cpuset='1-3'/>
 </cputune>
 <numatune>
   <memory mode='strict' placement='auto'/>
 </numatune>

We should not change the vcpu placement mode. Moreover, we're doing
something similar in case of emulatorpin and iothreadpin. If they were
set, but vcpu placement was auto, we've mistakenly removed them from
the domain XML even though we are able to set them independently on
vcpus.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>