iotests: Fix 104 for NBD

iotests: Fix 104 for NBD

_make_test_img sets up an NBD server, _cleanup_test_img shuts it down;
thus, _cleanup_test_img has to be called before _make_test_img is
invoked another time.

Furthermore, the pipe through _filter_test_img was unnecessary;
_make_test_img already takes care of that.

And finally, a filter is added to _filter_img_info to replace
"nbd://127.0.0.1:10810" by "TEST_DIR/t.IMGFMT", since the former is the
way to express the full image path (normally the latter) for NBD tests.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>