Update of the guest memory stats schema

Newer guest agent versions are now additionally reporting swap stats. This
needs to be reflected in the schema.

Change-Id: Ic6326ecac58e459cc73b87274cb18e8a3cf2ab41
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1025845
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20959
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>