qemuMigrationCookieNetworkXMLFormat: Refactor XML formatting
Use 'virXMLFormatElement' both for formating the whole <network> element
but also for formatting the <interface> subelements. This alows to
remove the crazy logic which was determining which element was already
formatted.
Additional simplification is achieved by switching to skipping the loop
using 'continue' rather than putting everything in a giant block.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>