https://bugzilla.redhat.com/show_bug.cgi?id=1459091

Currently, we are querying for vhostuser interface name in post
parse callback. At that time interface might not yet exist.
However, it has to exist when starting domain. Therefore it makes
more sense to query its name at that point. This partially
reverts 57b5e27d.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>