cfg: Fix tcpdump kill command to work on a wider variety of hosts

Using 'pkill -9 tcpdump' works on both new and older
distros.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

qemu/tests/cfg/migrate.cfg

@@ -3,7 +3,7 @@
     migration_test_command = help
     migration_bg_command = "cd /tmp; nohup tcpdump -q -i any -t ip host localhost"
     migration_bg_check_command = pgrep tcpdump
-    migration_bg_kill_command = pkill --signal 9 tcpdump
+    migration_bg_kill_command = pkill -9 tcpdump
     kill_vm_on_error = yes
     iterations = 2
     used_mem = 1024

qemu/tests/cfg/multi_host.cfg

@@ -17,7 +17,7 @@
             migration_test_command = help
             migration_bg_command = "cd /tmp; nohup tcpdump -q -i any -t ip host localhost"
             migration_bg_check_command = pgrep tcpdump
-            migration_bg_kill_command = pkill --signal 9 tcpdump
+            migration_bg_kill_command = pkill -9 tcpdump
             kill_vm_on_error = yes
             iterations = 2
             used_mem = 1024

qemu/tests/cfg/spice.cfg

@@ -43,7 +43,7 @@
             migration_test_command = help
             #migration_bg_command = "cd /tmp; nohup tcpdump -q -i any -t ip host localhost"
             #migration_bg_check_command = pgrep tcpdump
-            #migration_bg_kill_command = pkill --signal 9 tcpdump
+            #migration_bg_kill_command = pkill -9 tcpdump
             kill_vm_on_error = yes
             iterations = 2
             used_mem = 1024