virtio-blk: add num_request_queues module parameter

Sometimes a user would like to control the amount of request queues to
be created for a block device. For example, for limiting the memory
footprint of virtio-blk devices.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/20210902204622.54354-1-mgurtovoy@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>