conf: Allow usernet to have an address

conf: Allow usernet to have an address

https://bugzilla.redhat.com/show_bug.cgi?id=1075520

Currently, all that users can specify for an interface type of
'user' is the common attributes: PCI address, NIC model (and
that's basically it). However, some need to configure other
address range than the default one.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: laine@laine.org