vhost: handle virtually non-contiguous buffers in Tx

vhost: handle virtually non-contiguous buffers in Tx

This patch enables the handling of buffers non-contiguous in
process virtual address space in the dequeue path.

When virtio-net header doesn't fit in a single chunck, it is
copied into a local variablei before being processed.

For packet content, the copy length is limited to the chunck
size, next chuncks VAs being fetched afterward.

This issue has been assigned CVE-2018-1059.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>