qdev-properties-system.c: Allow vlan or netdev for -device, not both

It is currently possible to specify things like:
	-device e1000,netdev=foo,vlan=1
With this usage, whichever argument was specified last (vlan or netdev)
overwrites what was previousely set and results in a non-working
configuration.  Even worse, when used with multiqueue devices,
it causes a segmentation fault on exit in qemu_free_net_client.

That patch treates the above command line options as invalid and
generates an error at start-up.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
