tools/oxenstored: Avoid allocating invalid transaction ids

tools/oxenstored: Avoid allocating invalid transaction ids

The transaction id of 0 is reserved, meaning "not in a transaction".  It is up
to the xenstored server to allocate transaction ids.  While oxenstored starts
its ids at 1, but insufficient care is taken with truncation cases.

A 32bit oxenstored has an int with 31 bits of width, meaning that the
transaction id will wrap around to 0 after 2 billion transactions.

A 64bit oxenstored has an int with 63 bits of width, meaning that once 4
billion transactions are used, the allocated id will be truncated when written
into the uin32_t field in the ring.  This causes the client to reply with the
truncated id, breaking any further attempt to use any transactions.

Limit all transaction ids to the range between 1 and 0x7ffffffe.  This is the
best which can be done without making oxenstored depend on Stdint or Cstruct,
yet still work for 32bit builds.

Also check that the proposed new transaction id isn't currently in use.  For
the first 2 billion transactions there is no chance of a collision, and after
that, the chance is at most 20 (the default open transaction quota) in 2
billion.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: David Scott <dave@recoil.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>