implement O_NONBLOCK for /proc/xen/xenbus

This patch implements O_NONBLOCK for /proc/xen/xenbus.  It is a simple
matter of returning -EAGAIN instead of waiting on a queue.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>