common/blkdev: add simple sandboxing function for testing

Signed-off-by: Sage Weil <sage@redhat.com>
unittest_blkdev: test an abbreviated /sys/block dir

Note that we assume unittest_blkdev is run from the current directory (or
rather that test/common/test_blkdev_sys_block exists).

Signed-off-by: Sage Weil <sage@redhat.com>