the switch cases for the 4 different interface types had repetitive
code which has now been pulled out as common. While touching those
lines, some extra usage of "!= NULL" etc has been eliminated to make
things more compact and inline with current coding practices.
NB: parentIfType == VIR_INTERFACE_TYPE_LAST means that this is a
toplevel interface (not a subordinate of a bridge or bond). Only
toplevel interfaces can have a start mode, mtu, or IP address element.