tools/tests: Clean-up tools/tests/xen-access

tools/tests: Clean-up tools/tests/xen-access

The spin-lock implementation in the xen-access test program is implemented
in a fashion that is actually incomplete. The x86 assembly that guarantees that
the lock is held by only one thread lacks the "lock;" instruction.

However, the spin-lock is not actually necessary in xen-access as it is not
multithreaded. The presence of the faulty implementation of the lock in a non-
multithreaded environment is unnecessarily complicated for developers who are
trying to follow this code as a guide in implementing their own applications.
Thus, removing it from the code improves the clarity on the behavior of the
system.

Also converting functions that always return 0 to return to void, and making
the teardown function actually return an error code on error.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>