Add EdgeRef and ChildEdgeIteratorType to GraphTraits<DotNode *> and to
GraphTraits<const DotNode *>.
This allows iterating on edges of DotGraphs, which will be useful for
some testing on edge properties.
This commit replicates the structure of the install directory into the
build directory in order to simplify the management of RPATHs.
This commit also drop `QEMU_INSTALL_PATH`.