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: NJeff Cody <jcody@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>