Increase memory size of vm's to 2G

This is because ASAN bots autotests usually fail with out-of-memory.

More on the thread: https://groups.google.com/a/google.com/d/topic/chromeos-lab-infrastructure/Tuu4ir5Mc9c/discussion

BUG=chromium-os:24567
TEST=cbuildbot --local amd64-generic-asan -g I3b0f9156

Change-Id: I3b0f91566c45e0ef9e837995e353d7240ab9edea
Reviewed-on: https://gerrit.chromium.org/gerrit/24385
Commit-Ready: Denis Glotov <glotov@chromium.org>
Reviewed-by: Denis Glotov <glotov@chromium.org>
Tested-by: Denis Glotov <glotov@chromium.org>

lib/cros_vm_lib.sh

@@ -89,7 +89,7 @@ start_kvm() {
       cache_type="unsafe"
     fi
 
-    sudo kvm -m 1024 \
+    sudo kvm -m 2G \
       -smp 4 \
       -vga std \
       -pidfile "${KVM_PID_FILE}" \