core: fix ArithmeticException when adding host with no swap space

stat.getswap_total() will be divisor in checkVdsSwapThreshold(),
But when adding host which doesn't have swap space, it will return
zero

Change-Id: If4ad3d8b13cf2a784fcbb714434a676aba971102
Signed-off-by: plysan <plysab@gmail.com>