Disable PAT in linux VMs

Apparently kernel patch "x86/cpa: Use pte_attrs instead of pte_flags on
CPA/set_p.._wb/wc operations" (in out repo) doesn't fully solve the
problem and sometimes qubes-gui agent crashes with message like
"qubes-gui:664 map pfn expected mapping type write-back for [mem
0x00093000-0x00093fff], got uncached-minus".
Because PAT we really need only in dom0 (lack of it dramatically
decrease performance of some graphics drivers), we can simply disable it
in VM - as it is currently done in upstream kernel.

core/qubes.py

@@ -80,8 +80,8 @@ vm_files = {
 
 defaults = {
     'memory': 400,
-    'kernelopts': "",
-    'kernelopts_pcidevs': "iommu=soft swiotlb=4096",
+    'kernelopts': "nopat",
+    'kernelopts_pcidevs': "nopat iommu=soft swiotlb=4096",
 
     'dom0_update_check_interval': 6*3600,