iotests: implement QemuIoInteractive class

Implement QemuIoInteractive to test nbd-server-remove command when
there are active connections.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180119135719.24745-5-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>