qemu.tests.cfg: Use image_snapshot for enable snapshot for each guest

Use image_snapshot = yes to cfg file instead of set extra_params. To ignore
the problem that this parameter is set to no in other cfg files which will
corrupte the image.
Signed-off-by: NYiqiao Pu <ypu@redhat.com>

qemu/tests/cfg/cgroup.cfg

@@ -12,7 +12,7 @@
             vms = ""
             # problem with multiple preprocess, turn off the screendumps
             take_regular_screendumps = "no"
-            extra_params += " -snapshot"
+            image_snapshot = yes
             cgroup_test = "blkio_bandwidth"
             # cgroup_test_time, cgroup_weights, cgroup_limit{ ,_read,_write}
             # cgroup_weights = "[100, 1000, 500]"
@@ -21,7 +21,7 @@
             vms = ""
             # problem with multiple preprocess, turn off the screendumps
             take_regular_screendumps = "no"
-            extra_params += " -snapshot"
+            image_snapshot = yes
             cgroup_test = "blkio_throttle"
             # cgroup_test_time, cgroup_limit{ ,_read,_write}, cgroup_speeds
             # cgroup_speeds = [1024, 2048, 4096, 8192]
@@ -30,7 +30,7 @@
             vms = ""
             # problem with multiple preprocess, turn off the screendumps
             take_regular_screendumps = "no"
-            extra_params += " -snapshot"
+            image_snapshot = yes
             cgroup_test = "blkio_throttle_multi"
             # cgroup_test_time, cgroup_limit{ ,_read,_write}, cgroup_speeds
             # cgroup_speeds = "[[0, 1024, 0, 2048, 0, 4096],"
@@ -40,7 +40,7 @@
             vms = ""
             # problem with multiple preprocess, turn off the screendumps
             take_regular_screendumps = "no"
-            extra_params += " -snapshot"
+            image_snapshot = yes
             cgroup_test = "cpu_cfs_util"
             # cgroup_test_time, cgroup_limit
         - cpu_share:
@@ -48,13 +48,13 @@
             vms = ""
             # problem with multiple preprocess, turn off the screendumps
             take_regular_screendumps = "no"
-            extra_params += " -snapshot"
+            image_snapshot = yes
             cgroup_test = "cpu_share"
             # cgroup_use_max_smp, cgroup_test_time, cgroup_speeds
             cgroup_use_max_smp == 'yes'
             # cgroup_speeds = "[1000, 10000, 100000]"
         - cpuset_cpus:
-            extra_params += " -snapshot"
+            image_snapshot = yes
             cgroup_test = "cpuset_cpus"
             # cgroup_use_half_smp, cgroup_test_time, cgroup_limit, cgroup_cpuset, cgroup_verify
             cgroup_use_half_smp = 'yes'