conf: Keep 'readonly' property when resetting disk source

The property is necessary also for the disk using the source (e.g. cdrom)
which needs to be kept readonly.

Commit '462c4b66' was a bit too aggressive in this aspect, since the
readonly flag is set only while parsing.

src/conf/domain_conf.c

@@ -1723,9 +1723,12 @@ void
 virDomainDiskEmptySource(virDomainDiskDefPtr def)
 {
     virStorageSourcePtr src = def->src;
+    bool readonly = src->readonly;
 
     virStorageSourceClear(src);
     src->type = VIR_STORAGE_TYPE_FILE;
+    /* readonly property is necessary for CDROMs and thus can't be cleared */
+    src->readonly = readonly;
 }