KVM: Make unloading of FPU state when putting vcpu arch-independent

Instead of having each architecture do it individually, we
do this in the arch-independent code (just x86 as of now).

[avi: add svm to the mix, which was added to mainline during the
 2.6.24-rc process]

Signed-off-by: Amit Shah <amit.shah@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>