engine: fixed possible int overflow in getMemSharedPercent

- if there is system with 21TB+ shared memory the int value
will overflow when counting the percentage

Change-Id: I655a5558ff4fc7d7ad90ad5f9c4bd57ae9723838
Bug-Url: https://bugzilla.redhat.com/1093786
Signed-off-by: Jiri Moskovcak <moskovcak@gmail.com>