Add pre_command for some memory cases to drop cache before test

Signed-off-by: NYumei Huang <yuhuang@redhat.com>

qemu/tests/cfg/hotplug_mem.cfg

@@ -11,6 +11,7 @@
     maxmem_mem = 32G
     mem_devs = mem1
     login_timeout = 600
+    pre_command = "sync && echo 3 > /proc/sys/vm/drop_caches"
     no Host_RHEL.6
     no RHEL.5
     no Windows..i386

qemu/tests/cfg/migrate.cfg

@@ -10,6 +10,7 @@
     max_vms = 2
     mig_timeout = 3600
     ping_pong = 1
+    pre_command = "sync && echo 3 > /proc/sys/vm/drop_caches"
     # you can uncomment the following line to enable the state
     # check
     # vmstate_check = yes

qemu/tests/cfg/numa_memdev_options.cfg

@@ -11,6 +11,7 @@
     guest_numa_nodes = "node0 node1"
     numa_memdev_node0 = mem-mem0
     numa_memdev_node1 = mem-mem1
+    pre_command = "sync && echo 3 > /proc/sys/vm/drop_caches"
     variants:
         - prealloc:
             prealloc_mem0 = yes