vm-migration: Add MemoryFd command for setting FDs for memory

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-migration/src/protocol.rs

@@ -41,6 +41,7 @@ pub enum Command {
     Memory,
     Complete,
     Abandon,
+    MemoryFd,
 }
 
 impl Default for Command {
@@ -85,6 +86,10 @@ impl Request {
         Self::new(Command::Memory, length)
     }
 
+    pub fn memory_fd(length: u64) -> Self {
+        Self::new(Command::MemoryFd, length)
+    }
+
     pub fn complete() -> Self {
         Self::new(Command::Complete, 0)
     }