core: Allow Pools sorting according to missing fields

This patch makes it possible to sort search results according to
assigned and running pool pools count. This enhancement was needed as we
also want to sort by these fields in the ui.

Change-Id: Ia2e03d31e7bc67f750dcccfe0b57401f4630b13c
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1104790