Remove unnecessary minimum_version_id_old fields

Remove unnecessary minimum_version_id_old fields

The migration code will not look at a VMStateDescription's
minimum_version_id_old field unless that VMSD has set the
load_state_old field to something non-NULL.  (The purpose of
minimum_version_id_old is to specify what migration version is needed
for the code in the function pointed to by load_state_old to be able
to handle it on incoming migration.)

We have exactly one VMSD which still has a load_state_old,
in the PPC CPU; every other VMSD which sets minimum_version_id_old
is doing so unnecessarily. Delete all the unnecessary ones.

Commit created with:
  sed -i '/\.minimum_version_id_old/d' $(git grep -l '\.minimum_version_id_old')
with the one legitimate use then hand-edited back in.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
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It missed vmstate_ppc_cpu.