This allows the elimination of the spdk_event_get_arg1() and
spdk_event_get_arg2() macros, which accessed the event structure
directly; this was preventing the event structure definition from being
moved out of the public API header.
Change-Id: I74eced799ad7df61ff0b1390c63fb533e3fae8eb
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>