restapi: Do not cast memory.used to int

Casting the computed value to int caused an overflow
when the value was higher than about 4G. That is not
that unusual nowadays..

Change-Id: I216ac4ef5cf4cfd8cf523a84f75b252e0ce65ad0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1068884
Signed-off-by: Martin Sivak <msivak@redhat.com>