vduse: Validate vq_num in vduse_validate_config()

Add a limit to 'config->vq_num' which is user controlled data which
comes from an vduse_ioctl to prevent large memory allocations.

Micheal says  - This limit is somewhat arbitrary.
However, currently virtio pci and ccw are limited to a 16 bit vq number.
While MMIO isn't it is also isn't used with lots of VQs due to
current lack of support for per-vq interrupts.
Thus, the 0xffff limit on number of VQs corresponding
to a 16-bit VQ number seems sufficient for now.

This is found using static analysis with smatch.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Message-Id: <20221128155717.2579992-1-harshit.m.mogalapalli@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>