block: move aio initialization into a helper function Move AIO initialization for raw-posix block driver into a helper function. In addition to just code motion, the aio_ctx pointer is checked for NULL, prior to calling laio_init(), to make sure laio_init() is only run once. Signed-off-by: Jeff Cody <jcody@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>