core: allow 2% gap in host low swap

This patch change the computation of low swap memory such that it allows
a 2% gap between the configured low swap value
(LogPhysicalMemoryThresholdInMB) and the actual one in order to prevent
th engine log to be flooded with warning when the actual value is OK.

Change-Id: Ia3f5da5398141eeba83549a7e425e9f6b9f8673e
Signed-off-by: Eli Mesika <emesika@redhat.com>