Add "dmsetup ls --tree" output to lvmdump.

Add "dmsetup ls --tree" output to lvmdump.

It would be most useful to add "dmsetup ls --tree" to the commands run.
This command helps in answering the question "which devices are actually
underneath a given LV?"

Although the info is available with other existing dmsetup commands,
adding this command gives a much clearer summary of complex setups.

Here's an example of an LVM mirror, with mirror images on partitions
created on top of multipath devices.  The multipath devices are on
simple block devices.  As you can see, it is easy to see the stacking
from the "dmsetup ls --tree" output:

vgmpathtest-lvmpathmir (253:14)
 ├─vgmpathtest-lvmpathmir_mimage_1 (253:13)
 │  └─mpath5p1 (253:5)
 │     └─mpath5 (253:2)
 │        ├─ (8:16)
 │        └─ (8:0)
 ├─vgmpathtest-lvmpathmir_mimage_0 (253:12)
 │  └─mpath6p1 (253:6)
 │     └─mpath6 (253:3)
 │        ├─ (8:48)
 │        └─ (8:32)
 └─vgmpathtest-lvmpathmir_mlog (253:11)
    └─mpath7 (253:4)
       ├─ (8:80)
       └─ (8:64)
VolGroup00-LogVol01 (253:1)
 └─ (202:2)
vgtest-lvmir (253:10)
 ├─vgtest-lvmir_mimage_1 (253:9)
 │  └─ (7:1)
 ├─vgtest-lvmir_mimage_0 (253:8)
 │  └─ (7:0)
 └─vgtest-lvmir_mlog (253:7)
    └─ (7:3)
VolGroup00-LogVol00 (253:0)
 └─ (202:2)

But it is much harder to see the stacking with only the commands today
("dmsetup info", "dmsetup status", and "dmsetup table").  We could
piece together the stacking from "dmsetup table" but it requires
further processing (take output from "dmsetup info to get
map name to major/minor, then parse "dmsetup table", etc).