Allow printing special vdev metaslab groups

Sometimes, we'd like to know info about the metaslab groups
on special vdevs too. So let's make -MM do something useful.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12750