Revert "KVM: x86: drop fpu_activate hook"

This reverts commit 4473b570a7ebb502f63f292ccfba7df622e5fdd3.  We'll
use the hook again.

Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --cc arch/x86/include/asm/kvm_host.h
Simple merge

diff --cc arch/x86/kvm/svm.c
Simple merge

diff --cc arch/x86/kvm/vmx.c
Simple merge