Avoid use of uninitialized bufioreq_evtchn. It should only
be used if buffered IOREQs are enabled.
Resolves: Coverity CID 1563383
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>