iotests.py: Add qemu_nbd_pipe()

iotests.py: Add qemu_nbd_pipe()

In some cases, we may want to deal with qemu-nbd errors (e.g. by
launching it in a different configuration until it no longer throws
any).  In that case, we do not want its output ending up in the test
output.

It may still be useful for handling the error, though, so add a new
function that works basically like qemu_nbd(), only that it returns the
qemu-nbd output instead of making it end up in the log.  In contrast to
qemu_img_pipe(), it does still return the exit code as well, though,
because that is even more important for error handling.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20181221234750.23577-2-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>