vdpa: return errno in vhost_vdpa_get_vring_group error

vdpa: return errno in vhost_vdpa_get_vring_group error

We need to tell in the caller, as some errors are expected in a normal
workflow.  In particular, parent drivers in recent kernels with
VHOST_BACKEND_F_IOTLB_ASID may not support vring groups.  In that case,
-ENOTSUP is returned.

This is the case of vp_vdpa in Linux 6.2.

Next patches in this series will use that information to know if it must
abort or not.  Also, next patches return properly an errp instead of
printing with error_report.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20230526153143.470745-2-eperezma@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>