virtio-devices: vhost_user: classify disconnected backend errors

Add two explicit disconnected-backend error paths before wiring them
into the call sites.

MigratableError::DeviceDisconnected is the lifecycle sentinel for
operations that were skipped because a component is already known to be
disconnected. It lets the caller log and continue without treating it as
a VMM-fatal condition.

Error::BackendDisconnected is the vhost-user-local error used when
VhostUserCommon refuses to call a backend after its disconnected flag is
set. The transport classifier treats socket close/reset/EOF and
vhost-user partial-message/disconnected cases as transport loss, while
backend NACKs, invalid protocol state, and retry-able socket errors
remain ordinary operation failures.

Signed-off-by: Dylan Reid <dgreid@fb.com>

vm-migration/src/lib.rs

@@ -92,6 +92,9 @@ pub enum MigratableError {
 
     #[error("Failed to release a disk lock")]
     UnlockError(#[source] anyhow::Error),
+
+    #[error("Lifecycle operation skipped for disconnected component {0}")]
+    DeviceDisconnected(String),
 }
 
 /// A Pausable component can be paused and resumed.