net/passt: Remove dead code in passt_vhost_user_start error path (CID 1612371)

net/passt: Remove dead code in passt_vhost_user_start error path (CID 1612371)

In passt_vhost_user_start(), if vhost_net_init() fails, the "net"
variable is NULL and execution jumps to the "err:" label.

The cleanup code within this label is conditioned on "if (net)",
which can never be true in this error case. This makes the cleanup
block dead code, as reported by Coverity (CID 1612371).

Refactor the error handling to occur inline, removing the goto and
the unreachable cleanup block.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>