FPU: create lazy and non-lazy FPU restore functions

Currently Xen relies on #NM (via CR0.TS) to trigger FPU context restore. 
But not all FPU state is tracked by TS bit. This function creates two 
FPU restore functions: vcpu_restore_fpu_lazy() and 
vcpu_restore_fpu_eager(). vcpu_restore_fpu_lazy() is still used when #NM 
is triggered. vcpu_restore_fpu_eager(), as a comparision, is called for 
vcpu which is being scheduled in on every context switch.

Signed-off-by: Wei Huang <wei.huang2@amd.com>