https://bugzilla.redhat.com/show_bug.cgi?id=1366505

So far, this function lacked support for
VIR_DOMAIN_NET_TYPE_VHOSTUSER leaving callers to hack around the
problem by constructing the command line on their own. This is
not ideal as it blocks hot plug support.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>