conf: Make @def const in virDomainDefGetMemoryInitial

Keep const correctness and allow to use this function in cases where
@def is const in the caller.

src/conf/domain_conf.c

@@ -7796,7 +7796,7 @@ virDomainDefHasMemoryHotplug(const virDomainDef *def)
  * or the sum of memory sizes of NUMA nodes in case NUMA is enabled in @def.
  */
 unsigned long long
-virDomainDefGetMemoryInitial(virDomainDefPtr def)
+virDomainDefGetMemoryInitial(const virDomainDef *def)
 {
     return def->mem.initial_memory;
 }

src/conf/domain_conf.h

@@ -2325,7 +2325,7 @@ struct _virDomainDef {
     xmlNodePtr metadata;
 };
 
-unsigned long long virDomainDefGetMemoryInitial(virDomainDefPtr def);
+unsigned long long virDomainDefGetMemoryInitial(const virDomainDef *def);
 void virDomainDefSetMemoryTotal(virDomainDefPtr def, unsigned long long size);
 void virDomainDefSetMemoryInitial(virDomainDefPtr def, unsigned long long size);
 unsigned long long virDomainDefGetMemoryActual(virDomainDefPtr def);