virtio-blk: Fix integer overflow for large max IO sizes

virtio-blk: Fix integer overflow for large max IO sizes

When the maximum IO size supported by the virtio-blk backend is large
enough (>= 32MiB for 512B sectors), the computed blk_num_max will
overflow. In particular, if it's a multiple of 32MiB, blk_num_max
will end up as zero, causing IO requests to fail.

This is triggered by e.g. the SPDK virtio-blk vhost-user backend.

To fix it, just limit blk_num_max to 65535 before converting to u16.

Signed-off-by: Lukas Stockner <lstockner@genesiscloud.com>