s390x/kvm: synchronize guest floating point registers

Add code to kvm_arch_get_registers and kvm_arch_put_registers to
save/restore floating point registers. This missing sync was
unnoticed until migration of userspace that uses fprs.

Signed-off-by: Jason J. Herne <jjherne@us.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[Update patch to latest upstream]
Cc: qemu-stable@nongnu.org
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>