Use the current balloon size from libvirt info

This patch changes our current data source for balloon information
from internal vdsm balloon target to the real live data provided
by libvirt.

The old target value is now reported as balloon_target.

Change-Id: Id66d602da3a7c2306aad4363ef231a524ac6234c
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17109
Reviewed-by: Dan Kenigsberg <danken@redhat.com>