s390x: move fpu regs into a subsection of the vmstate

Let's move the floating point registers into a seperate subsection and
bump up the version id. This cleans up the current vmstate and will
allow for a future extension with vector registers in a compatible way.

This patch is based on a patch from Eric Farman.

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