lib: Move 'for_each_disk' function back into libvirt-domain.c.

This function is no longer called from anywhere outside the library,
or even outside libvirt-domain.c.

This is just code motion, except for the size_t changes which are
required in order to avoid signed overflow optimization error:

  assuming signed overflow does not occur when simplifying conditional
  to constant [-Werror=strict-overflow]