xen/vm_event: Decouple vm_event and mem_access.

The vm_event subsystem has been artifically tied to the presence of mem_access.
While mem_access does depend on vm_event, vm_event is an entirely independent
subsystem that can be used for arbitrary function-offloading to helper apps in
domains. This patch removes the dependency that mem_access needs to be supported
in order to enable vm_event.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
tools/tests: Remove superfluous and incomplete spinlock from xen-access

The spin-lock implementation in the xen-access test program is been 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-
mulithreaded environment is very confusing 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.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>