net: Move error reporting from net_init_client/netdev to the calling site

It looks strange that net_init_client() and net_init_netdev() both
take an "Error **errp" parameter, but then do the error reporting
with "error_report_err(local_err)" on their own. Let's move the
error reporting to the calling site instead to simplify this code
a little bit.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>