configure: Enable out-of-tree acceptance tests

configure: Enable out-of-tree acceptance tests

Currently to run Avocado acceptance tests in an out-of-tree
build directory, we need to use the full path to the test:

  build_dir$ avocado run /full/path/to/sources/qemu/tests/acceptance/boot_linux_console.py

This patch adds a symlink in the build tree to simplify the
tests invocation, allowing the same command than in in-tree builds:

  build_dir$ avocado run tests/acceptance/boot_linux_console.py

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180612173437.14462-1-f4bug@amsat.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>