conf: Ignore emulatorpin if vcpu placement is auto

conf: Ignore emulatorpin if vcpu placement is auto

When vcpu placement is "auto", the domain process will be pinned
to advisory nodeset from querying numad, While emulatorpin will
override the pinning. That means both of them are to set the
pinning policy for domain process, but conflicts with each other.

This patch ingore emulatorpin if vcpu placement is "auto", because
<vcpu> placement can't be simply ignored for <numatune> placement
could default to it.