net: do not exit on "netdev_add help" monitor command

"netdev_add help" is causing QEMU to exit because the code that
invokes show_netdevs is shared between CLI and HMP processing.
Move the check to the callers so that exit(0) remains only
in the CLI flow.

"netdev_add help" is not fixed by this patch; that is left for
later work.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>