Make net_client_init() consume slirp_configs even on error

Make net_client_init() consume slirp_configs even on error

net_slirp_init() walks slirp_configs, and stops when it encounters one
that doesn't work.  Instead of consuming slirp_configs members there,
consume them in the sole caller.  This makes sure all are consumed.
Before, the tail starting with the non-working one was left in place,
where it made the next net_slirp_init() fail again.

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