core: Fix balloon check so it supports more than 2TB of RAM

All BalloonInfo fields are Longs so it does not make sense
to compare them as Ints. Moreover the VM has more than about 2TB
of RAM then Int (32 bits) overflows.

Change-Id: I69c7ccf6d58cbf000d95e3448c75eb986ba39b0a
Signed-off-by: Martin Sivak <msivak@redhat.com>