qemu: drive_mirror - Rename variants

So it won't conflict with the existing reboot and powerdown
tests.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

qemu/tests/cfg/drive_mirror.cfg

@@ -80,11 +80,11 @@
                     before_start = "load_stress"
                     when_steady = "reopen"
                     after_reopen = "reboot verify_alive"
-        - reboot:
+        - with_reboot:
             type = drive_mirror_reboot
             when_steady = "reopen stop_reset"
             after_reopen = "reboot verify_alive"
-        - powerdown:
+        - with_powerdown:
             type = drive_mirror_powerdown
             app_check_cmd = "uname"
             start_cmd = 'sh -c "wget -c https://www.kernel.org/pub/linux/kernel/v2.6/longterm/v2.6.35/linux-2.6.35.14.tar.gz -O /mnt/Linux.tgz && '