vm-virtio: Add update_memory() to VirtioDevice trait

The virtio devices backed by a vhost-user backend must send an update to
the associated backend with the new file descriptors corresponding to
the memory regions.

This patch allows such devices to be notified when such update needs to
happen.

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

vm-virtio/src/lib.rs

@@ -175,4 +175,5 @@ pub enum Error {
     EpollCtl(io::Error),
     EpollWait(io::Error),
     FailedSignalingDriver(io::Error),
+    VhostUserUpdateMemory(vhost_user::Error),
 }