frontend: Make adding (virtio-)console to virtual machines optional

This patch makes it possible to control attaching virtio-console for
VMs/templates/VMs in pool.
The console is a managed device, therefore corresponding device type
was added. Also a command for querying DB for console devices was added.
Affected frontend behavior:
 - New checkbox in new/edit vm/template/pool dialog for controlling
   attaching of the console device.
 - For new vm/tepmlates entities if the operating system listbox
   changes, the console device checkbox is changed accordingly (if the
   selected OS is linux type, it is checked, otherwise it is unchecked).

Change-Id: Idf5f82b56940dbb43fe40f45a2a90eb004f29f4d
Bug-Url: https://bugzilla.redhat.com/878459
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>