This is most easily explained by the commit log of the first patch: Xen 4.2 become unsupported upstream in 09/2105 (see http://wiki.xen.org/wiki/Xen_Release_Features). However as far as the interfaces provided by the toolstack libraries go 4.2 and 4.3 are indistinguishable. Therefore drop support for Xen 4.1 and earlier which removes a whole pile of compatibility code which makes future work (to use stable library interfaces provided by upstream) more difficult. In particular all supported versions now use a pointer as a libxc handle (4.1 and earlier used an integer, resulting in various shim layers). Also Xen 4.2 was the first version of Xen to formally support upstream QEMU (as a preview) so that makes sense as a cut-off now.
The rest of the patches are just further cleanups which become possible.
