vmm: Add postcopy support to receive-migration

Plumb the SocketUffdMemorySource into the receiving side of live
migration. When memory_mode=postcopy is requested, the destination
brings up a dedicated fault connection, registers userfaultfd on the
restored memory regions, and serves guest pages on demand over that
connection while the VM resumes early.

Signed-off-by: Sebastien Boeuf <sboeuf@meta.com>
Assisted-by: Claude:claude-opus-4-7

vm-migration/src/lib.rs

@@ -47,6 +47,9 @@ pub enum UffdError {
     #[error("Region at {addr:#x}+{len:#x} missing COPY/WAKE support")]
     MissingIoctlSupport { addr: u64, len: u64 },
 
+    #[error("Failed to configure socket")]
+    SetSocket(#[source] io::Error),
+
     #[error("Failed to spawn handler thread")]
     SpawnThread(#[source] io::Error),