net: Fix bogus "Warning: vlan 0 with no nics" with -device

net_check_clients() prints this when an VLAN has host devices, but no
guest devices.  It uses VLANState members nb_guest_devs and
nb_host_devs to keep track of these devices.  However, -device does
not update nb_guest_devs, only net_init_nic() does that, for -net nic.

Check the VLAN clients directly, and remove the counters.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
