vdsm: Add support for VIR_MIGRATE_AUTO_CONVERGE parameter

Add support for configuring the VIR_MIGRATE_AUTO_CONVERGE migration
parameter in libvirt forcing convergence of given live migration.

This parmeter can be used when we want to guarantee convergence of
migration even at the cost of incremental slowing down of the guest.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1151001
Change-Id: I108e2480f5fcd6f80a209b9746c50aadcfc5c591
Signed-off-by: Martin Betak <mbetak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31991
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>