block/001: wait until device is added

block/001: wait until device is added

Writing to the scan attribute of scsi host is usually one sync scan, but
devices in this sync scan may be delay added if there is concurrent
asnyc scan.

So wait until the device is added in block/001 for avoiding to fail
the test.

Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
[Omar: fix quoting and simplify logic]
Signed-off-by: Omar Sandoval <osandov@fb.com>