In below example, before the fix, mtu attribute is supported
by the parent mgmtdev, but the error message showing "All
provided are not supported" is just misleading.

$ vdpa mgmtdev show
vdpasim_net:
  supported_classes net
  max_supported_vqs 3
  dev_features MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM

$ vdpa dev add mgmtdev vdpasim_net name vdpasim0 mtu 5000 max_vqp 2
Error: vdpa: All provided attributes are not supported.
kernel answers: Operation not supported

After fix, the relevant error message will be like:

$ vdpa dev add mgmtdev vdpasim_net name vdpasim0 mtu 5000 max_vqp 2
Error: vdpa: Some provided attributes are not supported: 0x1000.
kernel answers: Operation not supported

Fixes: d8ca2fa5 ("vdpa: Enable user to set mac and mtu of vdpa device")
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Message-Id: <1675725124-7375-2-git-send-email-si-wei.liu@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>