XenAPI: Increase OpenStack DomU usage again

Devstack has continued to grow in memory requirements and now we cannot
reliably fit in 4GB, with several services being unable to start.  Increase
the minimum for DomU to 6GB to leave room for virtual machines

Change-Id: Idbdfa1f36015b6af347d1ce27eb28baa360af5ef

Author: Bob Ball

tools/xen/xenrc

@@ -14,12 +14,12 @@ CLEAN_TEMPLATES=${CLEAN_TEMPLATES:-false}
 # Size of image
 VDI_MB=${VDI_MB:-5000}
 
-# Devstack now contains many components.  3GB ram is not enough to prevent
+# Devstack now contains many components.  4GB ram is not enough to prevent
 # swapping and memory fragmentation - the latter of which can cause failures
 # such as blkfront failing to plug a VBD and lead to random test fails.
 #
-# Set to 4GB so an 8GB XenServer VM can have a 1GB Dom0 and leave 3GB for VMs
-OSDOMU_MEM_MB=4096
+# Set to 6GB so an 8GB XenServer VM can have a 1GB Dom0 and leave 1GB for VMs
+OSDOMU_MEM_MB=6144
 OSDOMU_VDI_GB=8
 
 # Network mapping. Specify bridge names or network names. Network names may