migration: Rename MIG_EVENT_PRECOPY_* to MIG_EVENT_*

migration: Rename MIG_EVENT_PRECOPY_* to MIG_EVENT_*

All three events are shared between precopy and postcopy, rather than
precopy specific.

For example, both precopy and postcopy will go through a SETUP process.

Meanwhile, both FAILED and DONE notifiers will be notified for either
precopy or postcopy on completions / failures.

Rename them to make them match what they do, and shorter.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260126213614.3815900-6-peterx@redhat.com
[fixed-up entry in scsi-disk.c that got merged first]
Signed-off-by: Fabiano Rosas <farosas@suse.de>