conf: Fix the bug of disk->copy_on_read formating

The reason for it's not exposed for such long time is that the
enums for VirtioEventIdx and CopyOnReadType have same enum values
and Correspondingstrings. This fixes the bug and adds test.

tests/qemuxml2xmltest.c

@@ -293,6 +293,8 @@ mymain(void)
     DO_TEST("hostdev-scsi-virtio-scsi");
     DO_TEST("hostdev-scsi-readonly");
 
+    DO_TEST("disk-copy_on_read");
+
     virObjectUnref(driver.caps);
     virObjectUnref(driver.xmlopt);