tests/functional: Replace the ppc64 e500 advent calendar test

tests/functional: Replace the ppc64 e500 advent calendar test

Replace the advent calendar test with a buildroot image built with
qemu_ppc64_e5500_defconfig. Unlike the advent calendar image, this
newer buildroot image supports networking, too. Thus boot a ppce500
machine from kernel and disk, test network and poweroff.
Add '-no-shutdown' to the command line to avoid exiting from QEMU
as it seems to bother the functional framework.

Signed-off-by: Cédric Le Goater <clg@redhat.com>
Message-ID: <20250226065013.196052-1-clg@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[thuth: Add some wording about network support to the commit message]
Signed-off-by: Thomas Huth <thuth@redhat.com>