xenapi: increase default memory to 3G

Devstack was swapping with 2G

Change-Id: I8fe77591cb0ca0f946028d7219b43d77eea3419f
diff --git a/tools/xen/xenrc b/tools/xen/xenrc
index f698be1..6372ea7 100644
--- a/tools/xen/xenrc
+++ b/tools/xen/xenrc
@@ -13,7 +13,7 @@
 
 # Size of image
 VDI_MB=${VDI_MB:-5000}
-OSDOMU_MEM_MB=2048
+OSDOMU_MEM_MB=3072
 OSDOMU_VDI_GB=8
 
 # Network mapping. Specify bridge names or network names. Network names may