tools: Enable editing of maximum memory size of vm

This patch enables editing VM64BitMaxMemorySizeInMB and
VM32BitMaxMemorySizeInMB using engine-config.

Change-Id: Idb1e388db3e1fa9d72da9332f29d5a6e61720a40
Bug-Url: https://bugzilla.redhat.com/957729
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>