Don't try to build the ivshmem-server and ivshmem-client tools unless
CONFIG_IVSHMEM is set.

This fixes in passing a build bug on NetBSD, which fails to build the
ivshmem tools because they use shm_open() and on NetBSD that requires
linking against -lrt.

Signed-off-by: Kamil Rytarowski <n54@gmx.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1500021225-4118-4-git-send-email-peter.maydell@linaro.org
[PMM: moved some code into earlier patches; minor bugfixes;
 added commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>