virtio_mmio: fix logic

- Use separate descriptor count. Queue length and descriptor count do not need to be the same.

- Assotiate cookies with descriptors, not used queue index.

- Use mask instead of modulo operator for better efficiency (queue length is always power of 2).

Change-Id: I5f053287e2a0cfad8da25053c63c42aa06fb238d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6710
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

virtio_mmio: add new mmio interface

Change-Id: I2ca51d98e337d1e69a8e68d78ef0cfe33dc0d308
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3979
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>