network: populate interface pool immediately when network is started

network: populate interface pool immediately when network is started

When a network is defined with "<pf dev='xyz'/>", libvirt will query
sysfs to learn the list of all virtual functions (VF) associated with
that Physical Function (PF) then populate the network's interface pool
accordingly. This action was previously done only when the first guest
actually requested an interface from the network. This patch changes
it to populate the pool immediately when the network is started. This
way any problems with the PF or its VFs will become apparent sooner.

Note that we can't remove the old calls to networkCreateInterfacePool
that happen whenever a guest requests an interface - doing so would be
asking for failures on hosts that had libvirt upgraded with a network
that had been started but not yet used.

This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1047818