Fix "rmi" command from not removing everything

Commit 8e37354 ("Per-hypervisor images") broke "capstan rmi" to
accidentally remove all images from the local repository.  Fix it up.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Add test case for rmi command

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>