tests/qtest/migration: Define a machine for all architectures

tests/qtest/migration: Define a machine for all architectures

Stop relying on defaults and select a machine explicitly for every
architecture.

This is a prerequisite for being able to select machine types for
migration using different QEMU binaries for source and destination.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018192741.25885-8-farosas@suse.de>