tests/vhost-user-bridge: add scattering of incoming packets

This patch adds to the vubr test the scattering of incoming
packets to the chain of RX buffer.  Also, this patch corrects the
size of the header preceding the packet in RX buffers.

Note that this patch doesn't add the support for mergeable
buffers.

Signed-off-by: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>