block: test-iosched: enable running of simultaneous tests

Current test-iosched design enables running only a single test
for a single block device.
This change modifies the test-iosched framework to allow running
several tests on several block devices.

Change-Id: I051d842733873488b64e89053d9c4e30e1249870
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>