Another worthwhile split, needed one more public function.

* tools/virsh-nodedev.h: New file.
* tools/Makefile.am (virsh_SOURCES): Build it.
* tools/virsh-nodedev.c: Use new header.
* tools/virsh.c: Likewise.
(vshTreePrint): Export.
* tools/virsh.h (vshTreePrint): Declare.