engine: Tell VDSM the guaranteed amount of memory for VM

Give VDSM the data about the guaranteed memory size so it can
pass it to MoM and use it in the Balloon memory optimization.

Change-Id: If597a4be7a8cd1552c9c7c61175d8d6513c267ab
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=967574
Signed-off-by: Martin Sivak <msivak@redhat.com>