KVM: Allow kvm_load_guest_fpu() even when !vcpu->fpu_active

This allows accessing the guest fpu from the instruction emulator, as well as
being symmetric with kvm_put_guest_fpu().

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>