qemu: hotplug: Don't pretend that we support secrets for media change

qemu: hotplug: Don't pretend that we support secrets for media change

Old media changing code does not bother setting up the secrets for new
media or actually removing/adding of the corresponding objects.

Additionally it uses secrets setup for the old image to be removed as
the secret for the new image which is wrong.

Remove the support for secrets while changing media for the legacy
approach. The only reasonable way to fix it is when using blockdev.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>