vm-virtio: Add vhost-user-net implementation

vhost-user framwork could provide good performance in data intensive
scenario due to the memory sharing mechanism. Implement vhost-user-net
device to get the benefit for Rust-based VMMs network.

Signed-off-by: Cathy Zhang <cathy.zhang@intel.com>

vm-virtio/src/lib.rs

@@ -13,6 +13,7 @@ extern crate epoll;
 #[macro_use]
 extern crate log;
 extern crate pci;
+extern crate vhost_rs;
 extern crate virtio_bindings;
 extern crate vm_memory;
 
@@ -29,6 +30,7 @@ mod queue;
 mod rng;
 
 pub mod transport;
+pub mod vhost_user;
 
 pub use self::block::*;
 pub use self::console::*;
@@ -117,9 +119,16 @@ const INTERRUPT_STATUS_CONFIG_CHANGED: u32 = 0x2;
 pub enum ActivateError {
     EpollCtl(std::io::Error),
     BadActivate,
-
+    /// Queue number is not correct
+    BadQueueNum,
+    /// Failed to clone Kill event
+    CloneKillEventFd,
+    /// Failed to create Vhost-user interrupt eventfd
+    VhostIrqCreate,
     /// Failed to setup vhost-user daemon.
     VhostUserSetup(fs::Error),
+    /// Failed to setup vhost-user daemon.
+    VhostUserNetSetup(vhost_user::Error),
 }
 
 pub type ActivateResult = std::result::Result<(), ActivateError>;