virtio netdevice: optimize map_from_guest

Optimize map_from_guest to store the last found element on the
0-th position for faster finding later.

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
memory: optimize virtual_to_physical

A further optimization to virtual_to_physical.

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Merge pull request #193 from virtualopensystems/performance-v2

Performance optimizations targeting vhost-user