vmspawn-varlink: use error < 0 in async QMP completion callbacks

vmspawn-varlink: use error < 0 in async QMP completion callbacks

The QMP client always passes either 0 or a negative errno; the != 0 check
flagged values that cannot occur. Switch to the < 0 idiom used elsewhere
in the tree, and reorder on_qmp_simple_complete so the error path is the
first branch (the more conventional shape for callbacks). Equivalent in
behaviour.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>