vmm: api: Allow to delete non-booted VMs

The action of "vm.delete" should not report errors on non-booted
VMs. This patch also revised the "docs/api.md" to reflect the right
'Prerequisites' of different API actions, e.g. on "vm.delete" and
"vm.boot".

Fixes: #1110

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

vmm/src/lib.rs

@@ -424,8 +424,10 @@ impl Vmm {
             return Ok(());
         }
 
-        // First we try to shut the current VM down.
-        self.vm_shutdown()?;
+        // If a VM is booted, we first try to shut it down.
+        if self.vm.is_some() {
+            self.vm_shutdown()?;
+        }
 
         self.vm_config = None;