Don't call VM.stat every time we receive a xapi event on a VM.

VM.stat is expensive as it does lots of xenstore reads - just check that
the VM exists in xenopsd's database instead.

Signed-off-by: John Else <john.else@citrix.com>
Dump xenopsd updates every time we push an update.

Signed-off-by: John Else <john.else@citrix.com>
Sort events properly.

Signed-off-by: John Else <john.else@citrix.com>
CA-87377: Snapshot updates state in inject_barrier

Xapi then receives updates including barrier information from the
updates.get API call. Xapi processes the barriers first, as these
have threads waiting on them.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
CA-87377: Add barriers info to xn diagnostics

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>