V2: Use virAsprintf instead of snprintf/strdup

The xend driver will generate a virDomainNetDef ifname if one is not
specified in xend sexpr, even if domain is inactive.  The result is
network interface XML containing 'vif-1.Y' on dev attribute of target
element, e.g.

  <interface type='bridge'>
    <target dev='vif-1.0'/>
    ...

This patch changes the behavior to only generate the ifname if not
specified in xend sexpr *and* domain is not inactive (id != -1).