tests/avocado/boot_xen.py: fetch kernel during test setUp()

tests/avocado/boot_xen.py: fetch kernel during test setUp()

The kernel is a common blob used in all tests.  By moving it to the
setUp() method, the "fetch asset" plugin will recognize the kernel and
attempt to fetch it and cache it before the tests are started.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-ID: <20240806173119.582857-7-crosa@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-ID: <20240830133841.142644-3-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>