Change the target file name in live_backup test

This test uses "/home/test" by default which is usually present as a
directory. Let's use "/home/test.img" to decrease the probability of
existing directory.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/live_backup.cfg

@@ -24,7 +24,7 @@
             after_incremental += " verify_md5s verify_efficiency"
             Linux:
                 create_cmd = "dd if=/dev/urandom of=FILE bs=128k count=100"
-                file_names = "/home/test"
+                file_names = "/home/test.img"
                 test_exists_cmd = "test -f FILE"
             Windows:
                 create_cmd = "D:\coreutils\DummyCMD.exe FILE 1073741824 1"