powerpc/pseries: Move vas_migration_handler early during migration

When the migration is initiated, the hypervisor changes VAS
mappings as part of pre-migration event. Then the OS gets the
migration event which closes all VAS windows before the migration
starts. NX generates continuous faults until windows are closed
and the user space can not differentiate these NX faults coming
from the actual migration. So to reduce this time window, close
VAS windows first in pseries_migrate_partition().

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d8efade91dda831c9ed4abb226dab627da594c5f.camel@linux.ibm.com