network: unplug bandwidth and call networkRunHook only when appropriate

network: unplug bandwidth and call networkRunHook only when appropriate

According to commit b4e0299d if networkAllocateActualDevice() was
successful, it will *always* allocate an iface->data.network.actual,
so we can use this during networkReleaseActualDevice() to know if
there is really anything to undo. We were properly using this
information to only decrement the network connections counter if it
had previously been incremented, but we were unconditionally
unplugging bandwidth and calling the "unplugged" network hook for
*all* interfaces (during qemuProcessStop()) whether they had been
previously plugged or not. This caused problems if a domain failed to
start at some time prior to all interfaces being allocated. (I
encountered this when an interface had a bandwidth floor set but no
inbound QoS).

This patch changes both the call to networkUnplugBandwidth() and the
call to networkRunHook() to only be called if there was a previous
call to "plug" for the same interface.