tests/qtest: s390x: fix operation in a build without any boards or devices

tests/qtest: s390x: fix operation in a build without any boards or devices

Do the bare minimum to ensure that at least a vanilla
--without-default-devices build works for all targets except i386,
x86_64 and ppc64.  In particular this fixes s390x-softmmu; i386 and
x86_64 have about a dozen failing tests that do not pass -M and therefore
require a default machine type; ppc64 has the same issue, though only
with numa-test.

If we can for now ignore the cases where boards and devices are picked
by hand, drive_del-test however can be fixed easily; almost all tests
check for the virtio-blk or virtio-scsi device that they use, and are
already skipped.  Only one didn't get the memo; plus another one does
not need a machine at all and can be run with -M none.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240509170044.190795-6-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>