xenstored now supports multiple concurrent transactions per

xenstored now supports multiple concurrent transactions per
connection, plus interleaving of transactional and
non-transactional accesses. A transaction identifier is added
to the xsd_sockmsg header structure (0 means 'not in context
of a transaction'). The user and kernel xs interfaces accept
a pointer to a transaction handle where appropriate --
currently this is directly cast to an integer identifier in
the client library / kernel driver, but will allow for keeping
extra dynamic client-side state in future if we need to.

The transaction mutex has now gone. It's replaced with a
read-write mutex, but this is only acquired for exclusive
access during suspend/resume, to ensure there are no in-progress
transactions.

Signed-off-by: Keir Fraser <keir@xensource.com>