ide: Fix memory leak in ide_register_restart_cb()

Fix a memory leak in ide_register_restart_cb() in hw/ide/core.c and add
idebus_unrealize() in hw/ide/qdev.c to have calls to
qemu_del_vm_change_state_handler() to deal with the dangling change
state handler during hot-unplugging ide devices which might lead to a
crash.

Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1474995212-10580-1-git-send-email-ashijeetacharya@gmail.com
[Minor whitespace fix --js]
Signed-off-by: John Snow <jsnow@redhat.com>