x86/fpu: create lazy and non-lazy FPU restore functions

x86/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. To minimize restore overhead, it creates a flag,
nonlazy_xstate_used, to control non-lazy restore.

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