Fix domain events C example on Win32

Fix domain events C example on Win32

printf on Win32 does not necessarily support %lld and we don't
have GNULIBs wrapper for printf(). Switch to use asprintf() for
which we do have a gnulib wrapper with %lld support

* examples/domain-events/events-c/event-test.c: Fix formatting
  of %lld on Win32
* cfg.mk: Don't require use of virAsprintf since this is an
  example app for out of tree users to follow