qemu.tests: fix incorrect clean command for windows guest

Signed-off-by: NXu Tian <xutian@redhat.com>

qemu/tests/cfg/drive_mirror.cfg

@@ -82,6 +82,8 @@
                            type = drive_mirror_continuous_backup
                            clean_cmd = "rm -f tmp*.file"
                            dd_cmd = "dd if=/dev/zero bs=1024 count=1024 of=tmp%s.file"
+                           Windows:
+                               clean_cmd = "del /F tmp*.file"
         - with_stress:
             type = drive_mirror_stress
             reopen_timeout = 600