x86: Avoid calling xsave_alloc_save_area before xsave_init

x86: Avoid calling xsave_alloc_save_area before xsave_init

Currently, xsave_alloc_save_area will be called in
init_idle_domain->scheduler_init->alloc_vcpu->vcpu_initialise calls
with xsave_cntxt_size=0, it is earlier than xsave_init called in
identity_cpu(). This may causing buffer overflow on xmem_pool.

Idle domain isn't using FPU,SSE,AVX or any such extended state and
doesn't need it saved. xsave_{alloc,free}_save_area() should
test-and-exit on is_idle_vcpu(), and our context switch code should
not be doing XSAVE when switching out an idle vcpu.

Signed-off-by: Wei Gang <gang.wei@intel.com>
Signed-off-by: Keir Fraser <keir@xen.org>