fish: Add -c/--connect and -d/--domain options.

fish: Add -c/--connect and -d/--domain options.

The -d option lets you specify libvirt domains.  The disks from
these domains are found and added, as if you'd named them with -a.

The -c option lets you specify a libvirt URI, which is needed
when we consult libvirt to implement the above.