xenvm: evtchn: use proper barrier during events handling

This commit fixes barrier usage in Xen event channel driver. Previously
compiler_barrier() was used and it did not prevent processor from
re-ordering of the write operations, that is needed for correct event
handling. It is now changed to data memory barrier (dmb()), which will
work as expected in this situation.

Thanks @jgrall for suggestion.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>