userportal, webadmin: don't enable balloon on ppc arch

The memory ballooning doesn't work on ppc so neither portal
nor admin should allow to set it.

Change-Id: If5e6961778675807fb27e5ef4cb72209ee8762ef
Bug-Url: https://bugzilla.redhat.com/1127677
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>