vfio/migration: Add an Error** argument to vfio_migration_set_state()

vfio/migration: Add an Error** argument to vfio_migration_set_state()

Add an Error** argument to vfio_migration_set_state() and adjust
callers, including vfio_save_setup(). The error will be propagated up
to qemu_savevm_state_setup() where the save_setup() handler is
executed.

Modify vfio_vmstate_change_prepare() and vfio_vmstate_change() to
store a reported error under the migration stream if a migration is in
progress.

Reviewed-by: Avihai Horon <avihaih@nvidia.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>