cli: Support show by alias

Some collections, in particular the disks collection, don't support
retrieving elements by name, but by alias. This patch adds support for
this type of collections to the "show" command.

Change-Id: Ibf922ecf4d3f34a2c200ec7c145f18c15a8efd71
Bug-Url: https://bugzilla.redhat.com/1091725
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>