dom event example: init before register event impl

dom event example: init before register event impl

In the domain-events example C code virEventRegisterDefaultImpl was being
called before virConnectOpen without first calling virInitialize. While this
code worked, it is incorrect. Adding a call to g_string_new prior to the call
to virEventRegisterDefaultImpl would cause the code to break. This fix will
help avoid unintentional misue of the API.

Relates to: Ret Hat Bugzilla - Bug 961155