dev/xenstore: prevent transaction hijacking

The user-space xenstore device is currently lacking a check to make
sure that the caller is only using transaction ids currently assigned
to it. This allows users of the xenstore device to hijack transactions
not started by them, although the scope is limited to transactions
started by the same domain.

Tested by: Nathan Friess <nathan.friess@gmail.com>
Sponsored by: Citrix Systems R&D