vdpa/mlx5: Fix queue type selection logic

vdpa/mlx5: Fix queue type selection logic

get_queue_type() comments that splict virtqueue is preferred, however,
the actual logic preferred packed virtqueues. Since firmware has not
supported packed virtqueues we ended up using split virtqueues as was
desired.

Since we do not advertise support for packed virtqueues, we add a check
to verify split virtqueues are indeed supported.

Fixes: 707e0f8d9b65 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210811053759.66752-1-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>