Enable live migration only once the firmware supports PRE_COPY.

PRE_COPY has been supported by the firmware for a long time already [1]
and is required to achieve a low downtime upon live migration.

This lets us clean up some old code that is not applicable those days
while PRE_COPY is fully supported by the firmware.

[1] The minimum firmware version that supports PRE_COPY is 28.36.1010,
it was released in January 2023.

No firmware without PRE_COPY support ever available to users.
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20240306105624.114830-1-yishaih@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>