qemu: Reset image_extra_params parameter in some block tests.

These tests will create some other format of image, so the
options for qcow3 would affect them.
To make these cases still work with qcow3 format, reset the
'image_extra_params' parameter in them.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>

qemu/tests/cfg/multi_disk.cfg

@@ -23,6 +23,7 @@
             cdroms = ""
             ide:
                 # 3 + boot disk = 4 disks
+                image_extra_params = ""
                 stg_image_num = 3
                 stg_params  = "image_size:1G,10G,5G "
                 stg_params += "image_format:qcow2,qcow2,raw "

tests/cfg/block_hotplug.cfg

@@ -16,10 +16,12 @@
     kill_vm_on_error = yes
     variants:
         - fmt_qcow2:
+            image_extra_params = ""
             image_format_stg = qcow2
             image_format_stg0 = qcow2
             image_format_stg1 = qcow2
         - fmt_raw:
+            image_extra_params = ""
             image_format_stg = raw
             image_format_stg0 = raw
             image_format_stg1 = raw

tests/cfg/whql.cfg

@@ -203,6 +203,7 @@
                 - block_device:
                     image_snapshot = yes
                     images += " stg stg2 stg3 stg4"
+                    image_extra_params = ""
                     image_format_stg = raw
                     image_name_stg = images/storage
                     image_size_stg = 2G