iotests: add O_DIRECT alignment probing test

This test case checks that image files can be opened even if I/O
produces EIO errors.  QEMU should not refuse opening failed disks since
the guest may be configured for multipath I/O where accessing failed
disks is expected.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
