Clarify need for the migration_port Xen param

... depending on which toolstack is used.
Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

man/gnt-cluster.rst

@@ -334,10 +334,10 @@ configurable globally (at cluster level):
 migration\_port
     Valid for the Xen PVM and KVM hypervisors.
 
-    This options specifies the TCP port to use for live-migration. For
-    Xen, the same port should be configured on all nodes in the
-    ``@XEN_CONFIG_DIR@/xend-config.sxp`` file, under the key
-    "xend-relocation-port".
+    This options specifies the TCP port to use for live-migration when
+    using the xm toolstack. The same port should be configured on all
+    nodes in the ``@XEN_CONFIG_DIR@/xend-config.sxp`` file, under the
+    key "xend-relocation-port".
 
 migration\_bandwidth
     Valid for the KVM hypervisor.