Currently, an interface's vlan number corresponds to its index in
the table of network interfaces. That is no longer true when we
allow devices to be removed.

To fix this, we store the vlan number in the domain's state XML
so that it survives libvirtd restarts.

* src/domain_conf.h: add vlan number to virDomainNetDef

* src/domain_conf.c: store it in XML as <state vlan='N'/>, defaulting
  to -1 if this is state saved by a previous version of libvirt

* src/qemu_conf.c: assign vlan numbers before starting qemu