cli: Allow identifiers that aren't UUIDs

Currently we assume that the identifiers of objects are always UUIDs, but this
isn't true, as some objects like network labels use arbitrary strings as
identifiers, and the show commands doesn't work with them:

  [oVirt shell (connected)]# list labels \
    --network-identifier ovirtmgmt \
    --datacenter-identifier mydc

  id         : mylabel

  [oVirt shell (connected)]# show label \
    --network-identifier ovirtmgmt \
    --datacenter-identifier mydc \
    mylabel

  ============ ERROR ============
    label mylabel does not exist.
  ===============================

This patch removes that limitation so that the show command works correctly:

  [oVirt shell (connected)]# show label \
    --network-identifier ovirtmgmt \
    --datacenter-identifier mydc \
    mylabel

  id        : mylabel
  network-id: 4e31cd25-501e-47e4-a8b2-96b57a96a7e3

Change-Id: I2f3da6c3fd307ce9933c9d11d9ada4c4ff772e88
Bug-Url: https://bugzilla.redhat.com/1148941
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>