tools/oxenstored: Trim txhistory on xenbus reconnect

tools/oxenstored: Trim txhistory on xenbus reconnect

There is a global history, containing transactions from the past 0.05s, which
get trimmed whenever any transaction commits or aborts.  Destroying a domain
will cause xenopsd to perform some transactions deleting the tree, so that is
fine.  But I think that a domain can abuse the xenbus reconnect facility to
cause a large history to be recorded - provided that noone does any
transactions on the system inbetween, which may be difficult to achieve given
squeezed's constant pinging.

The theoretical situation is like this:
- a domain starts a transaction, creates as large a tree as it can, commits
  it. Then repeatedly:
    - start a transaction, do nothing with it, start a transaction, delete
      part of the large tree, write some new unique data there, don't commit
    - cause a xenbus reconnect (I think this can be done by writing something
      to the ring). This causes all transactions/watches for the connection to
      be cleared, but NOT the history, there were no commits, so nobody
      trimmed the history, i.e. it the history can contain transactions from
      more than just 0.05s
    - loop back and start more transactions, you can keep this up indefinitely
      without hitting quotas

Now there is a periodic History.trim running every 0.05s, so I don't think you
can do much damage with it.  But lets be safe an trim the transaction history
anyway on reconnect.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit 2a47797d1f3b14aab4f0368ab833abd311f94a70)