conf: add typedef for anonymous enum used for memballoon device model

For some reason the values of memballoon model are set using an
anonymous enum, making it impossible to perform nice tricks like
demanding there are cases for all possible values in a switch. This
patch turns the anonymous enum into virDomainMemballoonModel.

src/conf/domain_conf.h

@@ -1537,13 +1537,13 @@ struct _virDomainRedirFilterDef {
     virDomainRedirFilterUSBDevDefPtr *usbdevs;
 };
 
-enum {
+typedef enum {
     VIR_DOMAIN_MEMBALLOON_MODEL_VIRTIO,
     VIR_DOMAIN_MEMBALLOON_MODEL_XEN,
     VIR_DOMAIN_MEMBALLOON_MODEL_NONE,
 
     VIR_DOMAIN_MEMBALLOON_MODEL_LAST
-};
+} virDomainMemballoonModel;
 
 struct _virDomainMemballoonDef {
     int model;