I am not a fan of fixed-width buffers.  All it takes is a
linear chain of more than 100 snapshots to mess up 'virsh
snapshot-list --tree'.  Now that virBuffer is more powerful,
we might as well exploit its power.

* tools/virsh.c (cmdNodeListDevicesPrint): Simplify to use a
virBuffer instead of fixed-width prefix, factor guts, and rename...
(vshTreePrint, vshTreePrintInternal): ...along with new helper.
(cmdNodeListDevices, cmdSnapshotList): Update callers.