powerpc: Fix running the kvm-unit-tests with recent versions of QEMU

Starting with version 7.0, QEMU starts the pseries guests in 32-bit mode
instead of 64-bit (see QEMU commit 6e3f09c28a - "spapr: Force 32bit when
resetting a core"). This causes our test_64bit() in powerpc/emulator.c
to fail. Let's switch to 64-bit in our startup code instead to fix the
issue.

Message-Id: <20221206110851.154297-1-thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>