While running objecteventtest, it was found that valgrind pointed out the
following memory leak:

==13464== 5 bytes in 1 blocks are definitely lost in loss record 7 of 134
==13464==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
==13464==    by 0x341F485E21: strdup (strdup.c:42)
==13464==    by 0x4CAE28F: virStrdup (virstring.c:554)
==13464==    by 0x4CF3CBE: virObjectEventCallbackListAddID (object_event.c:286)
==13464==    by 0x4CF49CA: virObjectEventStateRegisterID (object_event.c:729)
==13464==    by 0x4CF73FE: virDomainEventStateRegisterID (domain_event.c:1424)
==13464==    by 0x4D7358F: testConnectDomainEventRegisterAny (test_driver.c:6032)
==13464==    by 0x4D600C8: virConnectDomainEventRegisterAny (libvirt.c:19128)
==13464==    by 0x402409: testDomainStartStopEvent (objecteventtest.c:232)
==13464==    by 0x403451: virtTestRun (testutils.c:138)
==13464==    by 0x402012: mymain (objecteventtest.c:395)
==13464==    by 0x403AF2: virtTestMain (testutils.c:593)
==13464==

Hitting this should be pretty rare, but at least developers will know
that they are providing a weird event ID. Otherwise for namespace that
are added in the normal way, gcc will raise a warning about unhandled
case in the switch.

Define the public API for (de-)registering network events
and the callbacks for receiving lifecycle events. The lifecycle
event includes a 'detail' parameter to match the domain lifecycle
event data, but this is currently unused.

The network events related code goes into its own set of internal
files src/conf/network_event.[ch]