qdev: Revert the hack to let -net nic and pci_add set qdev ID

Setting the ID in pci_nic_init() is a blatant violation of the
DeviceState abstraction.  Which even carries a comment advising
against this:

/* This structure should not be accessed directly.  We declare it here
   so that it can be embedded in individual device state structures.  */

What's worse, it bypasses the code ensuring unique qdev IDs: "-device
virtio-net-pci,id=foo -net nic,id=foo -net nic,name=foo" happily
creates three qdevs with ID "foo".  That's because qdev relies on
qemu_opts_create() to ensure unique IDs, but -net nic uses a different
QemuOptsList, which means id is in a different namespace.  And its
name is not checked for uniqueness at all.

-net nic and pci_add are legacy.  Use -device and device_add if you
want a NIC with a qdev ID.

This reverts what's still left of commit eb54b6dc "qdev: add id=
support for pci nics."

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