virtio: add support for SIZE_MAX & SEG_MAX features

virtio: add support for SIZE_MAX & SEG_MAX features

Some virtio implementations may forward the virtio requests directly to
the underlying hw. The hw may have some restrictions in how many and how
big the requests can be. Therefore, the corresponding virtio device will
announce this limitations with the SIZE_MAX & SEG_MAX feature.

Add support for those features. Split an io request into multiple virtio
requests if more than seg_max segments would be used. Also split a
single buffer request into multiple segments if the buffer is bigger
then size_max.

Signed-off-by: Christian Pötzsch <christian.poetzsch@kernkonzept.com>
Signed-off-by: Adam Lackorzynski <adam@l4re.org>