board: xen: Remove CONFIG_CMD_RUN define and clean xenguest_arm64_defconfig

CONFIG_CMD_RUN is set on by default in Kconfig.
Create xenguest_arm64_defconfig by using savedefconfig to avoid unnecessary
options.

Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
Reviewed-by: Simon Glass <sjg@chromium.org>