vdsm: Add support for VIR_MIGRATE_COMPRESSED parameter

Add support for configuring the VIR_MIGRATE_COMPRESSED migration
parameter in libvirt to compress repeated pages.

This flag enables the XBZRLE compresson of same memory pages which is
beneficial for certain workloads, e.g. database server with many
duplicate data in memory.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1151001
Change-Id: I5391e7966dd46d7b33962fb3adf2830381472247
Signed-off-by: Martin Betak <mbetak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31992
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>