docker: test-block: Don't continue if build fails

Report error and exit upon compiling error, otherwise the iotests output
will be pure noise.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170926110134.2786-1-famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

tests/docker/test-block

@@ -14,7 +14,7 @@
 
 cd "$BUILD_DIR"
 
-build_qemu --target-list=x86_64-softmmu
+build_qemu --target-list=x86_64-softmmu || test_fail "Build failed"
 cd tests/qemu-iotests
 for t in raw qcow2 nbd luks; do
     ./check -g quick -$t || test_fail "Test failed: iotests $t"