engine: Search VMs, Templates, and Storage by description

Users have asked for the ability to search for Virtual Machines by
description.  Add 'description' as a condition field for all objects which
have a description field in the Database but do not yet support it for
search: VMs, Storage, and VM Templates.

Change-Id: I2909b3444cf6729680018150974a644d2b61a790
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=526471
Signed-off-by: Adam Litke <alitke@redhat.com>