Calling qsort() on the disks array causes disk to be
unneccessarily re-ordered, potentially breaking the
ability to boot if the boot disk gets moved later in
the list. The new algorithm will insert a new disk as
far to the end of the list as possible, while being
ordered correctly wrt other disks on the same bus.

* src/domain_conf.c, src/domain_conf.h: Remove disk sorting
  routines. Add API to insert a disk into existing list at
  the optimal position, without resorting disks
* src/libvirt_private.syms: Export virDomainDiskInsert
* src/xend_internal.c, src/xm_internal.c: Remove calls to
  qsort, use virDomainDiskInsert instead.
* src/qemu_driver.c: Remove calls to qsort, use virDoaminDiskInsert
  instead. Fix reordering bugs when hotunplugging disks and
  networks. Fix memory leak in disk/net unplug