ACRN: dm: Enable mount namespace for container.

The patch adds mount namespace for the container. Without the patch
when we destroy the container, there still have mountinfo for the
container. This is one workaroud for runC bug. If the mount namespace
is disabled, when try to mount host "/" to container "/", there will
cause the issue. Detail discussion as following link.
"https://groups.google.com/a/opencontainers.org/forum/#!searchin/
dev/mount$20namespace%7Csort:date/dev/p10bq-kXODk/obkqBRdxCQAJ"

After enable mount namespace, some small performance imapcts for
specific mediator, likes USB. Will keep monitor runc community,
and revert this patch once related runc patches get integrated.

Tracked-On: #2020
Signed-off-by: Long Liu <long.liu@intel.com>
Acked-by: Yu Wang <yu1.wang@intel.com>

devicemodel/samples/apl-mrb/runC.json

@@ -265,6 +265,9 @@
 			},
 			{
 				"type": "uts"
+			},
+			{
+				"type": "mount"
 			}
 		]
 	}