Update virNetDevMacVLanCreateWithVPortProfile to allow creation
of plain macvlan devices, as well as macvtap devices. The former
is useful for LXC containers

* src/qemu/qemu_command.c: Explicitly request a macvtap device
* src/util/virnetdevmacvlan.c, src/util/virnetdevmacvlan.h: Add
  new flag to allow switching between macvlan and macvtap
  creation