iotests: add 208 nbd-server + blockdev-snapshot-sync test case

iotests: add 208 nbd-server + blockdev-snapshot-sync test case

This test case adds an NBD server export and then invokes
blockdev-snapshot-sync, which changes the BlockDriverState node that the
NBD server's BlockBackend points to.  This is an interesting scenario to
test and exercises the code path fixed by the previous commit.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20180306204819.11266-3-stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>