network: don't allow multiple default portgroups

network: don't allow multiple default portgroups

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.