vmm: memory_manager: Make the snapshot source directory an Option

This allows the code to be reused when creating the VM from a snapshot
when doing VM migration.

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

vmm/src/lib.rs

@@ -397,7 +397,7 @@ impl Vmm {
             &snapshot,
             exit_evt,
             reset_evt,
-            source_url,
+            Some(source_url),
             restore_cfg.prefault,
             &self.seccomp_action,
             self.hypervisor.clone(),