tests/migration-tests: Always enable migration events

tests/migration-tests: Always enable migration events

Libvirt should always enable it, so it'll be nice qtest also cover that for
all tests on both sides.  migrate_incoming_qmp() used to enable it only on
dst, now we enable them on both, as we'll start to sanity check events even
on the src QEMU.

We'll need to leave the one in migrate_incoming_qmp(), because
virtio-net-failover test uses that one only, and it relies on the events to
work.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>