ZTS: Fix list_file_blocks for mirror vdevs, level > 0

ZTS: Fix list_file_blocks for mirror vdevs, level > 0

The first part of list_file_blocks transforms the pool configuration
output by zdb -C $pool into shell code to set up a shell variable,
VDEV_MAP, that maps from vdev id to the underlying vdev path. This
variable is a simple indexed array. However, the vdev id in a DVA is
only the id of the top level vdev.

When the pool is mirrored, the top level vdev is a mirror and its
children are the mirrored devices. So, what we need is to map from
the top level vdev id to a list of the underlying vdev paths.
ist_file_blocks does not need to work for raidz vdevs, so we can
disregard that case.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11141