Use min_guarantee to set the minimum guaranteed memory available

Changes in this patch should ensure the minimal memory guarantee
needed for a rudimentary memory QoS support.

This change requires the following VDSM patches:
 http://gerrit.ovirt.org/15799
 http://gerrit.ovirt.org/16774

Change-Id: I376ac6b2cdadc567314899a95c6b855f820e746b
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15801
Reviewed-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-by: Adam Litke <agl@us.ibm.com>
Tested-by: Adam Litke <agl@us.ibm.com>