s390x: Migrate vector registers

When migrating a guest, be sure to include the vector registers.
The vector registers are defined in a subsection, similar to the
existing subsection for floating point registers.  Since the
floating point registers are always present (and thus migrated),
we can skip them when performing the migration of the vector
registers which may or may not be present.

Suggested-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>