Delete the format limit for some cases

Support some cases both for luks and luks-inside-qcow2.
Signed-off-by: NTingting Mao <timao@redhat.com>

qemu/tests/cfg/luks_compare.cfg → qemu/tests/cfg/qemu_img_compare.cfg

-- luks_compare:
-    only luks
+- qemu_img_compare:
     virt_test_type = qemu
     type = luks_convert
     start_vm = no

qemu/tests/cfg/rebase_second_snapshot_to_base.cfg

 - rebase_second_snapshot_to_base:
-    only raw, luks
     virt_test_type = qemu
     type = rebase_second_snapshot_to_base
     kill_vm = yes