core: [Fix] Disabled Balloon in Add Vm

Currently, it is impossible to add a VM with balloon disabled
(REST/UI), this change fixes this bug.

Change-Id: I508ff77c1e309be0b492f46c31311e03f33fc728
Bug-Url : https://bugzilla.redhat.com/show_bug.cgi?id=1049343
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>