iotests: Make 087 pass without AIO enabled

If AIO has not been enabled in the qemu build that is to be tested, we
should skip the "aio=native without O_DIRECT" test instead of failing.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171115180732.31753-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/087

@@ -102,7 +102,14 @@ echo
 echo === aio=native without O_DIRECT ===
 echo
 
-run_qemu <<EOF
+# Skip this test if AIO is not enabled in this build
+function run_qemu_filter_aio()
+{
+    run_qemu "$@" | \
+        sed -e 's/is not supported in this build/it requires cache.direct=on, which was not specified/'
+}
+
+run_qemu_filter_aio <<EOF
 { "execute": "qmp_capabilities" }
 { "execute": "blockdev-add",
   "arguments": {