Merge remote-tracking branch 'sstabellini/for_anthony' into staging

* sstabellini/for_anthony:
  xen: introduce an event channel for buffered io event notifications
  xen-mapcache: don't unmap locked entry during mapcache invalidation
  Xen, mapcache: Fix the compute of the size of bucket.
  xen: handle backend deletion from xenstore
  Xen: Add xen-apic support and hook it up.
  Xen: basic HVM MSI injection support.