Read the message type out of the message before sending it to xenstored, and

Read the message type out of the message before sending it to xenstored, and
use that saved value when handling the reply.  Xenstored will leave the
message type intact, _except_ when returning an error, in which case it will
change the type to XS_ERROR.  This meant that we failed to remove a
transaction from our internal list if xenstored returned EAGAIN, as we did not
realise that the message was XS_TRANSACTION_END.  This manifested itself as
the intended behaviour until the connection was closed, at which point all of
those failed transactions would erroneously be aborted.

Signed-off-by: Ewan Mellor <ewan@xensource.com>