ls-tree: Fix parsing devices on multidomain PCI system

ls-tree: Fix parsing devices on multidomain PCI system

Represent each domain as domain bridge under the &host_bridge and put root
bus of each domain under the domain bridge.

With this change lspci in tree view does not show zero bus on domain 0 in
the output if this bus does not exist at all. Root bus in PCIe hierarchy
does not have to be zero and on Freescale PowerPC systems it is common.

Also with this change are separate domain showed in the output separately.