Increase default number of CPUs to 4

Increasing parallelism might increase our chances of catching
concurrency bugs, eg. in page table modifications.

Change-Id: Ieb9695d7a3f4daca9dbc1f877d99ad118ef9f1d3
diff --git a/aarch64/run_qemu.sh b/aarch64/run_qemu.sh
index 9ffea8c..924177b 100755
--- a/aarch64/run_qemu.sh
+++ b/aarch64/run_qemu.sh
@@ -22,7 +22,7 @@
 default_var ROOTFS		"${PREBUILTS_KUT_ROOTFS}"
 default_var TEMPLATE		""
 default_var CPU			"max"
-default_var SMP			2
+default_var SMP			4
 default_var RAM			512
 default_var GIC			3
 default_var GDB			0