Allow user to specify name for new export, to not reuse internal
node name and to not show it to clients.

This also allows creating several exports per device.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180119135719.24745-2-vsementsov@virtuozzo.com>
Signed-off-by: NEric Blake <eblake@redhat.com>