[vhost] fix vhost_user_map_guest_memory and vhost_user_unmap_guest_memory

The size argument of these functions should be uint64_t instead of int.
Currently it causes issues with large guest RAM sizes (2G).

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
[virtio] support non-zero copy case

We have to properly handle the case when the zero copy is not properly 
initialised, i.e. vhost_user:rx_buffers() is not used as a source 
of RX buffers. When this is detected just get a buffer from rx_buffers()
and copy everything in it.

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Merge pull request #174 from virtualopensystems/non-zero-copy

Handle non zero copy case in virtio device