history: Changed mem_shared column type and name

Removed casting of mem_shared column, and it will now
be of int type. Also, changed the ending of the column name
to "mb" instead of "percent", because the data is in mb.

Change-Id: Ia3a42581d00168d091659d30ebfbc086b1757695
Bug-Url: https://bugzilla.redhat.com/1076902
Signed-off-by: Shirly Radco <sradco@redhat.com>