Make net_client_init() accept addr=, put the value into struct
NICinfo.  Use it in pci_nic_init(), and remove arguments bus and
devfn.

Don't support addr= in third argument of monitor command pci_add,
because that clashes with its first argument.  Admittedly unelegant.

Machines "malta" and "r2d" have a default NIC with a well-known PCI
address.  Deal with that the same way as the NIC model: make
pci_nic_init() take an optional default to be used when the user
doesn't specify one.

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