macvtap: cannot support target device name

macvtap: cannot support target device name

Since the macvtap device needs active tear-down and the teardown logic
is based on the interface name, it can happen that if for example 1 out
of 3 interfaces was successfully created, that during the failure path
the macvtap's target device name is used to tear down an interface that
is doesn't own (owned by another VM).

So, in this patch, the target interface name is reset so that there is
no target interface name and the interface name is always cleared after
a tear down.