core, engine: Architecture parameter on search backend

This change adds a search parameter used to filter the architecture of
VMs, Templates, Clusters, Pools and Hosts. In other words, the user can
show only entities with a specific architecture.

Change-Id: Ib02d2136e58bee976fdbb1833de17ad724615283
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>