net: netmap: use error_setg() helpers in place of error_report() This update was required to align error reporting of netmap backend initialization to the modifications introduced by commit a30ecde. Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com> Signed-off-by: Jason Wang <jasowang@redhat.com>