This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=868483
virNetworkUpdate, virNetworkDefine, and virNetworkCreate all three
allow network definitions to contain multiple <portgroup> elements
with default='yes'. Only a single default portgroup should be allowed
for each network.
This patch updates networkValidate() (called by both
virNetworkCreate() and virNetworkDefine()) and
virNetworkDefUpdatePortGroup (called by virNetworkUpdate() to not
allow multiple default portgroups.
(cherry picked from commit 6f8a8b30c9a0123d8c6f49c946084b94c580811b)
10f8762b4f