qemu_command: replace vlan= with netdev= for legacy nic
    
    VMs with hardcoded platform network devices are forced to use old
    style '-net nic' command line config. Current we use qemu's vlan
    option to hook this with the '-netdev' host side of things.
    
    However since qemu 1.2 there is '-net nic,netdev=X' option for
    explicitly referencing a netdev ID, which is more inline with
    typical VM commandlines, so let's switch to that
    
    Signed-off-by: Cole Robinson <crobinso@redhat.com>