docker: Add test_fail and prep_fail

They both print a message and exit, but with different status code so
distinguish real test errors from env preparation failures.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170905025614.579-3-famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Based-on: 20170905021201.25684-1-famz@redhat.com

tests/docker/common.rc

@@ -32,3 +32,15 @@ build_qemu()
     echo $config_opts
     $QEMU_SRC/configure $config_opts && make $MAKEFLAGS
 }
+
+test_fail()
+{
+    echo "$@"
+    exit 1
+}
+
+prep_fail()
+{
+    echo "$@"
+    exit 2
+}