vmm: Handle ApiRequest::VmCreate in a separate function

It simplifies a bit the `Vmm::control_loop` and reads better to be
consistent with other `ApiRequest` handlers. Also, it removes the
repetitive `ApiError::VmAlreadyCreated` and makes `ApiError::VmCreate`
useful.

No functional changes.

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

vmm/src/api/mod.rs

@@ -64,9 +64,6 @@ pub enum ApiError {
     /// The VM could not boot.
     VmBoot(VmError),
 
-    /// The VM is already created.
-    VmAlreadyCreated,
-
     /// The VM could not be created.
     VmCreate(VmError),