Allow disk operation to act on a subset of disks

If the disks= parameter is passed, we can assemble/wait for
sync/shutdown only some disks belonging to an instance, rather than all.

This is useful to only activate/sync/shutdown the affected disk when
growing it.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>