All the callers of these functions only check for a negative
return value.

However, virNetDevOpenvswitchGetVhostuserIfname is documented
as returning 1 for openvswitch interfaces so preserve that.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>