virtio: Do not scale desc_needed by _num / 2

There is no reason we should do the scale.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Dor Laor <dor@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
virtio: Fix vring::use_indirect

When the _avail_count is less than 1/3 of the ring, we start using
indirect descriptor.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Dor Laor <dor@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>