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/vm.rs

@@ -680,7 +680,7 @@ impl Vm {
         snapshot: &Snapshot,
         exit_evt: EventFd,
         reset_evt: EventFd,
-        source_url: &str,
+        source_url: Option<&str>,
         prefault: bool,
         seccomp_action: &SeccompAction,
         hypervisor: Arc<dyn hypervisor::Hypervisor>,