conf: don't ignore <target dev='blah'/> for macvtap interfaces
    
    The parser had been clearing out *all* suggested device names for
    type='direct' (aka macvtap) interfaces. All of the code implementing
    macvtap allows for a user-specified device name, so we should allow
    it. In the case that an interface name starts with "macvtap" or
    "macvlan" though, we do still clear it out, just as we do with "vnet"
    (which is the prefix used for automatically generated tap device
    names), since those are the prefixes for the names we autogenerate for
    macvtap and macvlan devices.
    
    Resolves: https://bugzilla.redhat.com/1335798