virt: repair problem with undefined cpu_model

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds possibility for forbid preprocess_env for multihost mig.

Useful for multihost migration when VM is already started.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds multihost cdrom tests.

Signed-off-by: Jiří Župka <jzupka@redhat.com>