vmm: api: Refactor generic action handler

Rather than save the save a function pointer and use that instead the
underlying action. This is useful for two reasons:

1. We can ensure that we generate HttpErrors in the same way as the
   other endpoints where API error variant should be determined by the
   request being made not the underlying error.
2. It can be extended to handle other generic actions where the function
   prototype differs slightly.

As result of this refactoring it was found that the "vm.delete" endpoint
was not connected so address that issue.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/api/http.rs

@@ -34,6 +34,9 @@ pub enum HttpError {
     /// Could not boot a VM
     VmBoot(ApiError),
 
+    /// Could not delete a VM
+    VmDelete(ApiError),
+
     /// Could not get the VM information
     VmInfo(ApiError),