Add support for swap information to be used in policy

This patch adds support for swap size and swap usage values to
be collected by guest agent or qemu agent and used in memory
calculations.

The same variables are also collected from Host and used
in pressure detection algorithm.

Change-Id: If1f8ae4d090e27a41ee541acc9398cd1a56b459a
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20978
Reviewed-by: Adam Litke <agl@us.ibm.com>