common: support dm-delay to introduce I/O delays

common: support dm-delay to introduce I/O delays

Add some infrastructure in common/dmdelay to support use of the
dm-delay device-mapper module within tests. This is effectively
copied from the existing infrastructure in common/dmflakey. This
provides the ability to delay I/O. It only supports delaying read
I/O as that is all that is required at this point in time.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>