vmm: Only build a new VM when booting it

In order to support further use cases where a VM configuration could be
modified through the HTTP API, we only store the passed VM config when
being asked to create a VM. The actual creation will happen when booting
a new config for the first time.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vmm/src/api/mod.rs

@@ -52,6 +52,12 @@ pub enum ApiError {
     /// The VM could not boot.
     VmBoot(VmError),
 
+    /// The VM is already created.
+    VmAlreadyCreated,
+
+    /// The VM config is missing.
+    VmMissingConfig,
+
     /// The VM could not shutdown.
     VmShutdown(VmError),