dmflakey: support multiple dm targets for a dm-flakey device

dmflakey: support multiple dm targets for a dm-flakey device

dm-flakey can be used to emulate IO write error, however, when we
also need to prevent the IO error for a specific range of the block
device (eg., the log region of a XFS), we need to specify multiple
dm targets for the dm device.

Option --table can not accommodate the multiple dm targets case, so
let dmsetup get the possible-multiple-targets table from standard
input.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>