vmm: Refactor slightly vm_boot and 'control_loop'

It ensures all handlers for `ApiRequest` in `control_loop` are
consistent and minimum and should read better.

No functional changes.

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/vm.rs

@@ -142,6 +142,9 @@ pub enum Error {
     /// Failed to join on vCPU threads
     ThreadCleanup(std::boxed::Box<dyn std::any::Any + std::marker::Send>),
 
+    /// VM config is missing.
+    VmMissingConfig,
+
     /// VM is not created
     VmNotCreated,