Fix crash changing CDROM media

Fix crash changing CDROM media

This change tried to fix a crash with changing CDROM media but
failed to actually do so

  commit d0172d2b1b5d865aaa042070d7c2d00effb2ff8c
  Author: Osier Yang <jyang@redhat.com>
  Date:   Tue Feb 19 20:27:45 2013 +0800

    qemu: Remove the shared disk entry if the operation is ejecting or updating

It was still accessing disk->src, when the entire 'disk' object
has been free'd already. Even if it weren't free'd, accessing
the 'src' value of virDomainDiskDef is not allowed without
first validating disk->type is file or block. Just remove the
broken code entirely.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>