core: Enable balloon by default - Patch 1 of 2

Some guest OSes may crash if the VirtIO balloon is enabled in ppc64
VMs. This can be worked around by the system administrator if he
disables the balloon kernel module in the guest OS.
This change introduces the following modifications:
* Adds in the osinfo file a property to indicate if the balloon device
  must be disabled by default:
  os.{id}.devices.balloon.enabled.value = false
* Changes the OS repository classes to work with the new property.
* Modifies the REST API in order to disable the balloon device if the
  Memory Policy is omitted in cases where the guest OS has this new
  propery enabled.

Change-Id: I1b048b3d54e5e1732f8a13de8c9d4c1df4b6d8e5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1069858
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>