This patch removes the expanded and duplicated code that all sprung
out of two well-intentioned-but-useless settings of
net->data.(bridge|ethernet).ipaddr.

qemu has never supported even a single IP address in the interface
config, much less a list of them. All of the instances of "clearing
out the IP addresses" that are now in this function originated with
commit d8dbd6 "Basic domain XML conversions for Xen/QEMU drivers" in
May 2009, but even then the single "ipaddr" in the struct for
type='ethernet' and type='bridge' wasn't used in the qemu driver (only
in xen and openvz). Since then anyone who added a new interface type
also tacked on another unnecessary clearing of ipaddr, and when it was
made into a list of IPs (so far supported only by the LXC driver) this
simple setting was turned into a loop (well, multiple loops) to clear
them all.