[libcheckers] use async I/O for directio checker

[libcheckers] use async I/O for directio checker

The directio checker will block until the request is returned.
We should rather use async I/O to guarantee that the checker
will return after a certain time so as not to stall the entire
daemon.

Signed-off-by: Stefan Bader <bader@de.ibm.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christophe Varoqui <christophe.varoqui@free.fr>