vmm: Create vsock backend

This commit relies on the new vsock::unix module to create the backend
that will be used from the virtio-vsock device.

The concept of backend is interesting here as it would allow for a vhost
kernel backend to be plugged if that was needed someday.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/vsock/mod.rs

@@ -14,6 +14,8 @@ mod packet;
 mod unix;
 
 pub use self::device::Vsock;
+pub use self::unix::VsockUnixBackend;
+pub use self::unix::VsockUnixError;
 
 use std::os::unix::io::RawFd;