vdpa/mlx5: Support creating resources with uid == 0

Currently all resources must be created with uid != 0 which is essential
when userspace processes are allocating virtquueue resources. Since this
is a kernel implementation, it is perfectly legal to open resources with
uid == 0.

In case firmware supports, avoid allocating user context.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210531160404.31368-1-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>