net: Reject non-netdevs in qmp_netdev_del()

The netdev_del command crashes when given a -net device, because it
calls qemu_opts_del(NULL).

Check that this is a -netdev before attempting to delete it and the
QemuOpts.

Note the subtle change from qemu_find_opts_err("netdev", errp) to
qemu_find_opts_err("netdev", NULL).  Since "netdev" is a built in
options group and we don't check for NULL return anyway, there's no use
in passing errp here.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
