virDomainDiskGetSource: Mark passed disk as 'const'

The disk is not changed anywhere in the function. Mark this fact
in the function header too.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/conf/domain_conf.c

@@ -1467,7 +1467,7 @@ virDomainDiskSetType(virDomainDiskDefPtr def, int type)
 
 
 const char *
-virDomainDiskGetSource(virDomainDiskDefPtr def)
+virDomainDiskGetSource(virDomainDiskDef const *def)
 {
     return def->src->path;
 }

src/conf/domain_conf.h

@@ -2450,7 +2450,7 @@ void virDomainDiskDefFree(virDomainDiskDefPtr def);
 void virDomainLeaseDefFree(virDomainLeaseDefPtr def);
 int virDomainDiskGetType(virDomainDiskDefPtr def);
 void virDomainDiskSetType(virDomainDiskDefPtr def, int type);
-const char *virDomainDiskGetSource(virDomainDiskDefPtr def);
+const char *virDomainDiskGetSource(virDomainDiskDef const *def);
 int virDomainDiskSetSource(virDomainDiskDefPtr def, const char *src)
     ATTRIBUTE_RETURN_CHECK;
 const char *virDomainDiskGetDriver(virDomainDiskDefPtr def);