qmp_pmemsave: update test case steps, add check the size of pmemsave

Signed-off-by: Njinzhao <jinzhao@redhat.com>

qemu/tests/cfg/qmp_command.cfg

@@ -62,9 +62,9 @@
             event_cmd_type = host_cmd
             qmp_cmd = "pmemsave val=0, size=4096, filename=/var/tmp/pmemsave"
             pre_cmd = "rm -rf /var/tmp/pmemsave"
-            post_cmd = "ls /var/tmp/pmemsave; true"
-            cmd_result_check = post_contain
-            cmd_return_value = pmemsave
+            post_cmd = "stat -c %s /var/tmp/pmemsave"
+            cmd_result_check = "post_equal"
+            cmd_return_value = 4096
         - qmp_query-cpus:
             qmp_cmd = "query-cpus"
             post_cmd = "info cpus"