conf: don't ignore <target dev='blah'/> for macvtap interfaces

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