core: Enable balloon by default - Patch 2 of 2

The "New VM"/"New Pool" dialogs in the frontend were changed to enable
the balloon device in guest OSes where the "balloon.enabled" osinfo
property is enabled. This way the system administrator will have an
usable VM by default even in problematic guest OSes.

Change-Id: I7803e45f07539090e5527ebc81bbc6c31df4e120
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1069858
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>
Signed-off-by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>